The goal of all of them is to fire a flash that's not on the camera.

Let's take a look at what all this means

• radio trigger (e.g. PocketWizard) They talk to each other via radio, which means there's no flash firing just for communication purposes.

  • You will definitely need a sender unit on the camera, because no camera (as of yet) has built in sender. This is because the triggers are third party solutions. Not even Canon, who have speedlights with built in radio receiver have a sender in any camera model.

  • In general, you also need a receiver on each flash unit. Some higher end studio strobes come with built in PocketWizard receivers or radio receivers of their own system (you usually also get a sender for the camera)

  This has the advantage of working in situations where line of sight is not possible. The communication works better around corners and in general over bigger distances.

  • You can have "dumb" communication that basically tells the slave units to fire.

  • Or you can have full TTL via radio.

  Different systems provide different feature sets.

• "slave mode" This term is a bit confusing in this context. The radio trigger communication also requires a master to trigger several slave units. What you are referring to is the communication built into the flash unit itself. Most flashes can be set to be slave.

  • The flash unit has a little light sensor to detect if another flash fires. This is a "dumb" slave mode, which fires as soon as it sees the other flash. A dumb slave will work across systems (fire FUQ690 flash with Buzz Lightyear camera ;) ) There are "smarter" slaves that will wait a certain amount of flashes, before firing. This allows them to be used in conjunction with TTL flashes. The TTL flashes will fire a bunch of times to get their communication done. The "smart" slave know how many flashes the communication takes an fires after that, in sync with the other flashes. Both smart and dumb slaves are just firing the flash, there are no groups or metering, whatsoever. You pretty much replace pushing the fire/test button on the unit or closing the middle pin on its socket.

  • If you pair a flash unit and camera of one manufacturer, chances are the camera can act as a master for the remote slave units either via its pop-up flash or another flash unit on-camera. This allows you to do TTL: The slaves and the master talk to each other and figure out the best flash output power. But there's more. At the end of the whole TTL dialog, each flash will fire with a certain power setting. If you take away the communication, that means this also allows you to remotely dial in a manual value for each unit. And there usually is exactly such an option: set the power output level of each flash unit. This is very useful, because you do not have to go to each slave unit to change its setting. Or maybe you have a unit on a stand up high above. If you take it down to change the power, chances are you will not be able to set it up in the same hight and orientation. You'd have to fiddle around a bit to get it right again, which can be undesirable. This option usually means groups for flashes, TTL as mentioned, ratios of groups, etc.

• cable We live in a wireless world. That's why we curse when the wifi is slow because a dozen people share it with us. Or we cannot make a phone call because the building blocks the signal. Cables do not have this problem. A cable is the most reliable, most cheap way to trigger a remote flash. There are cables to make "dumb" slaves but also advanced cables that allow TTL.

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Confused? Yeah, it's a bit of a mess. Here's a conclusion:

What do the flashes use to communicate? radio, infrared line of sight or cable

What are they communicating? dumb signal "fire now", TTL, remote manual control and/or advanced stuff like rear curtain sync or readiness of the slave to fire

What is compatible with what? dumb remotes work with pretty much everything across different brands, anything more advanced will usually only work with one brand or if third party manufacturers reverse engineer the system to be compatible to that brand specific system