A quick summary of the possible ways to trigger a flash. Note that not all trigger methods support TTL flash (many are Manual flash only).
All studio flash units are manual flash only (not integrated into any camera's own system). Camera speedlights may offer either or both Manual and TTL flash modes. The difference between triggering manual flash and TTL flash is a very huge difference. TTL requires a proprietary communication from the camera system. The PC sync cord, optical slave, and most radio systems are manual flash mode only.
(automatic TTL flash is something else - see TTL section, below)
All studio lights are manual flash, since they don't know the rules of the special camera TTL systems. One manual speedlight on the hot shoe can easily be tweaked using rear LCD image and/or histogram to check results and retry. Multiple manual lights in serious setups really need a hand held flash meter to set each light's level relative to the others. Manual flash is really what gives us our full control, since we can set each light exactly the way we want it. Most speedlights offer Manual mode, and these triggering accessories below can connect to a remote speedlight flash unit via the PC sync connector, or via a foot mounted accessory adapter, of the types below.
PC Sync Cable:
You can add a hot shoe adapter to add a PC sync connector to a camera which does not have one (a very handy substitute on a camera with PC too, unless you have a PC cord with Screw-Lock, to be tighter in camera PC). The Nikon AS-15 is a particularly good one, has the clamp to keep it from sliding off center. For speedlights with no PC connector, there are third party PC adapters which can be added to their foot. Then you can use a PC Male to PC Male sync cable (long ones are getting hard to find) from the camera to the flash. Studio lights have a different connector at their end, generally provide the right cable. It is good advice to always have a spare PC cable handy, as these can eventually go bad and fail now and then (as can anything) - Not often, but always at the worst possible time. Flash meters also have a PC connector, so you can use the same cord that fits your light to the flash meter, to remotely trigger and meter the flash.
The older Nikon three-pin cables SC-18/SC-19 and SC-26/SC-27 versions (functionally identical), were called TTL cables, but this means film TTL, NOT iTTL (multiple iTTL remotes are wireless, iTTL does not use any cables). If the older flash model has the connection, your three-pin cable can still be used today for manual flash mode (not the way to buy today though). I use a three-pin cable Between two SB-800, one of which is triggered by a PC cable, to trigger two manual flashes in a tabletop situation.
The PC sync cord might be seen as low tech today, but simplicity is a good thing too. The cord causes very little trouble on a fixed tripod-mounted camera, to the nearest light - which is probably the fill light near the camera, aimed at the other lights, which probably have optical slave sensors on them. This works great in a studio situation.
A PC Male to PC Female cable is an extension cord for any PC cord, so you could, for example, use a 12 inch PC Male to PC Male cord with a 15 foot PC Male to PC Female extension cord. The connectors can pull apart easily (accidental disconnection), so there are versions with screw threads for the camera or flash ends. Between two cords, you could tie them in a knot at the connector, to prevent any stress.
This PC sync cord method is only a simple trigger, so it is only Manual operation, no TTL. There are three-way Y adapters to split and run cables to multiple flashes, but you would probably prefer to run a sync cable to the nearest flash, and its flash will trigger the other flashes equipped with optical slave triggers.
Hot-shoe Extension Cable:
For the Nikon system, these are the Nikon SC-17, SC-28, SC-29 cables (and there are cheaper non-Nikon brands too). These cables simply extend all the hot shoe pins to the remote flash - an extension cord. On this hot shoe extension cord, the flash can use any mode it can do on the hot shoe, iTTL or Manual flash, but the cable only controls one flash unit, or a commander. These cables work well with a hand held flash, just about right to hold at arms length. The most common use seems to be on a flash bracket above the camera (these cords allow TTL operation, where a PC sync cord cannot). These are coiled cords, and may technically stretch longer, however they are practically limited to about four feet (enough for an umbrella near the camera), because if stretched longer, the coil tension pulls back strongly enough to probably tip over your light stand. The loose stand sitting on a table is NOT going to work this way, you will have to anchor it down well (if a coiled cord under tension).
The older Nikon SC-17 cable still works fine on newer cameras and flash units, and it is plentiful and inexpensive on Ebay. This older SC-17 is the same cable as the newer Nikon SC-28 TTL Remote Cord, except it just does not have the pin hole for the hot shoe lock of the new flashes (which the new SC-28 does have). The old SC-17 still works fine however, just lower the pin anyway (it is spring loaded) and the flash will not fall out, or you can drill that hole yourself. I went years without the hole in my old SC-17 cables, and then finally got around to drilling it. It really does not matter. SC-17 is inexpensive, plentiful on Ebay, and it works fine, it is all I have. The SC-28 is "the same" except it has the pin lock in the shoe for the newer flashes. And, the new cord exits the camera end to the side, instead of forward.
The Nikon SC-29 TTL Off-Camera Shoe Cord with AF Assist has an additional AF Focus Assist light at the camera end. The speedlight already has a focus assist light to help focus in dim light, however when off camera, it probably is not be aimed at the same focus point the camera needs. The SC-29 adds another focus assist light in the hot shoe, which will always be aimed directly forward, where the camera needs it. This assist is only needed in the nearly dark places when focus is not otherwise possible. If it is otherwise bright enough for the camera to be able to focus, then it is not needed.
You can chain up to three of these cables to be a little longer. Each is an extension cord. Or, there are other brand cords which are longer. Or, you can heat the old (inexpensive, expendable) SC-17 in hot water, and two of you can tug to straighten out the coils into be an almost straight cable perhaps 8 feet long. It may be a bit kinky and ugly, but it should work well. Some long third party cables are available.
Popular outdoors, or on hand held cameras.
A radio transmitter module goes in the camera hot shoe, and a receiver module attaches to the flash, often via PC sync connector, or many models use a shoe foot connector for speedlights. Radio has advantage of relatively great range, will go through/around most obstacles, and is not affected by bright sunlight - so radio is only choice in some extreme cases. Involves expense, and a few extra parts to deal with, extra connections, extra batteries. Receiver battery requirement is high, because the receiver always must be listening.
Radio triggers are a mixed bag. There are expensive ones that are very reliable, and there are many less expensive imports that may be a little less reliable (about triggering every time). Virtually all are simple Manual flash mode triggers, but Radio Popper and Pocket Wizard each have one model that simply relay the Nikon commander system via radio, with the same features and issues then, except radio is not limited in the way of optical triggers.
Very popular in the studio.
An optical slave is built-in on virtually all studio lights, and accessories can be added to the foot of speedlights. This is just a photo-detector, which senses the flash of an another manual light, and triggers the connected flash in sync. Works quite well in the indoor studio. This one first light must be triggered some other way, anyway we can trigger it (usually sync cord or radio trigger), and then its flash triggers the other slaves in sync (all lights are in Manual flash mode).
The optical slave trigger can be an added accessory, either added at the flashes PC sync connector, or many fit the flash shoe foot, which triggers the flash in sync when the light from another manual flash is detected. Some other Manual mode flash does have to be triggered some other way, to trigger them. Most studio lights include this optical slave trigger built-in, and they routinely operate this way. We must trigger the first nearest light some other way, like with a sync cord or radio trigger (or it could be the camera internal flash set to minimum Manual power level), and then the light from the first one triggers all the others in sync. Manual flash mode works, including Nikon of course, but the Nikon commander mode above is VERY different, and incompatible with Manual mode, in that the Commander uses a special complex trigger signal flashes at minimum power level, which will trigger optical slaves too early. But this optical slave is triggered from the regular full working power flash from any other manual flash, so reliability can be very good, indoors. Range is very reasonable but not extreme, and these work great in a studio environment. Outside, distance and sunlight can be problems. Downside is that any other photographer using flash in same room will trigger your lights too. Manual flash only.
Optical slave triggers are incompatible with digital TTL flash mode. The digital preflash will trigger them too early to contribute, before the shutter opens. Generally, all flashes involved must be in manual flash mode. However, there are "digital" slave triggers which can ignore the hot shoe digital TTL preflash, BUT these still CANNOT ignore the commander flashing (still incompatible with Commander).
SB-600 performance has been spotty with some added optical triggers, because the slave triggers contain no battery, and are powered by the sync voltage of the flash unit. No additional batteries to watch is a good thing, but the SB-600 sync voltage is about one volt lower than usual, a little low for this, and slave performance cannot be predicted (most work, some don't). In that regard, see the testing of a few Optical Slave Triggers from Ebay.
Nikon SU-4 mode:
The SB-700, SB-800, SB-900, SB-910 already contain excellent built in optical slave triggers (called SU-4 mode).
The Nikon SU-4 flash mode is simply a very good built-in optical slave trigger. The actual Nikon SU-4 Wireless Remote Slave TTL Flash Controller is a shoe-foot optical slave trigger device (which can be added to speedlights without a slave). The Nikon SB-700 to SB-910 flashes (NOT the SB-600 or SB-400) include this internally, with a menu to set them into SU-4 mode, where they become an optically triggered slave, such that the light from any Manual flash will trigger them. They will work well with studio lights this way, they become just another optically triggered slave flash. The older Nikon SB-26, SB-50DX and SB-80DX models also include a builtin internal optical slave trigger (manuals here).
One flash does have to be triggered to flash some other way, in order to trigger the optical slaves. This could be your pop-up flash, set to Manual mode, and set to lowest manual power - to not affect your picture much, if at all. This low power pop-up is not going to do much to your picture, except at very close subject distances. The main flash will be much stronger. The SU-4 mode flashes are quite sensitive, and 1/128 power from 60 feet away will trigger them, IF their sensor is pointing at the source. Probably more, but 60 feet is the longest I have tried. In an indoor studio situation, with the high power main light triggering them, random wall reflections work well and line of sight seems no issue. This is Manual flash mode, no TTL. There is an Auto mode, where the flash meters and controls the flash itself, if the ISO and aperture used by the camera is specified.
Examples - Multiple flash Manual systems:
Very common solutions for indoor studio flash are:
All of the methods above are for Manual flash mode. TTL requires more.
Hot Shoe Flash
The hot shoe works of course, including optional use of a hot shoe extension cord (Nikon SC-28, SC-29, or older SC-17 hot shoe cables) for one single iTTL flash only. The digital flash will preflash immediately before the shutter opens, which the camera TTL system meters, and sets the flash power level via the hot shoe. Note that any use of a PC sync cord or optical trigger or SU-4 remote flash will only work in Manual flash mode, but not with any other iTTL flash. iTTL control simply must be connected to the hot shoe.
Bounce Flash with Hot Shoe flash
Bounce is hot shoe flash, not exactly off-camera of course, but it is not direct flash either. A speedlight fits the camera hot shoe, which then triggers with the shutter button, in either digital iTTL or manual flash mode. The hot shoe is not the best location for a flash, but it can be very effective there too, particularly if used as bounce flash from ceiling or wall. Bounce flash eliminates the flat frontal lighting of direct flash (sort of like off camera). Bounce flash is an easy thing to do to improve your flash, and is also among the best things... Bounce should always be your standard method for a hot shoe flash, if there is any suitable ceiling or wall present.
Nikon Commander/Remote Wireless Remote System
The Commander system is the only means to control multiple iTTL flashes. All other iTTL is one flash only, connected to the hot shoe. The one commander flash unit can optionally be on a Hot Shoe extension cord (Nikon SC-28, SC-29, or older SC-17). Otherwise, there simply is no concept of any cable in the iTTL system.
The Nikon Commander could be the internal popup flash of camera models with commander. Nikon D40/D50/D60 and D3100/D5100 do not include a commander, but models D80/D90/D200/D300/D7000/D700 do. Or the commander could be an added $250 Nikon SU-800, or a SB-700 through SB-910 flash (on the hot shoe, operating as Commander). The commander must be on the camera, or attached to it with a hot shoe extension cord.
The commander flash can also contribute to your lighting, or not. Your choice, the commander menu has a "- -" choice to disable that internal flash. Even when disabled, the commander WILL FLASH commands before the shutter opens. With the commander and a couple of remote flashes, this is a very impressive system, allowing automatic wireless remote point&shoot TTL multiple flash. The Commander can do iTTL or manual power levels. Much preflashing occurs which causes subject blinking, but camera FV Lock is a work around for TTL (however camera models without commander do not implement FV Lock). Manual power levels should NOT be confused with Manual flash mode - it is still the commander, and incompatible with real Manual flash mode, with a few downsides not encountered with real Manual flash mode. See a similar discussion of the differences of speedlights vs studio lights.
We cannot mix and match Manual flash gear with the Nikon Commander system. Digital camera flash is simply very different than the old days. Optical slaves are simply incompatible with all the preflashing done by the commander system (a Sekonic flash meter can be used with Commander manual mode, **IF** the commander is the SU-800, or if the Nikon SG-3IR IR Panel for Camera Built-In Flashes infrared panel is used on a camera commander) -- and **IF** the meter is not closer than 5 or 6 feet to the infrared commander (may get EU errors if closer). The preflashing will trigger your slaves too early, before the shutter opens, and thus they cannot contribute to your picture lighting. You can use the Commander system, or you can use the Manual system. Just pick one system and go with it, then no problems.
Film Cameras - The film camera watched the TTL light on the film surface in real time, and quenched the flash off when exposure was sufficient. The quench pin on the flash foot is how the flash is controlled. In contrast, digital TTL cannot do that, and it must trigger and meter a preflash, and then set the flash power setting accordingly. This is a very big difference in the way remote flashes can be controlled, the equipment becomes incompatible.
The Nikon three-pin cables SC-18/SC-19 and newer SC-26/SC-27 versions (functionally identical), are called TTL cables, but this specifically means Film TTL, which used these cables for multiple TTL flashes. These cables can only do Manual flash with digital iTTL cameras, which do NOT use cables for multiple iTTL flash. The speedlight versions for film TTL had these three pin connectors, and the Nikon AS-10 multiple flash adapter and the SC-17/SC-28/SC-29 hot shoe extension cords are sources with this three pin connector, again, which can only be used for Manual flash if on digital iTTL systems. These TTL cables merely quenched (terminated) the remote flash when the camera quenched the hot shoe flash. Film TTL mode has no metering or control of individual remote flashes.
Older TTL methods, including film TTL and digital D-TTL (as opposed to current iTTL methods):
The SU-4 controlled remote flashes was a multiple TTL flash system for film TTL. The SU-4 remotes watched and "followed" the hot shoe flash, which was a controlled TTL exposure, and they quenched their flash when the hot shoe flash stopped. This did control maximum flash exposure, but it offered no control of the intensity of the individual flashes. You had to place them at the right distance.
There is no multiple D-TTL flash. The SU-4 remotes could be tried, but the digital preflash will also trigger the remotes (too early). The Nikon flashes (if set to lower power, but not at full power) were generally fast enough that they could trigger both times, by the preflash and again by final working flash, and this trigger "could" still work. It is very far from a knowledgeable elegant try however.