Wireless trigger types
Optical
An optical wireless trigger fires when it detects a pulse of visible light, so can be triggered with your pop-up flash, or another flashgun, either on- or off-camera. That makes them very easy to use as they require no physical setup. The downside is you have to have line-of-sight between the light source and the optical wireless trigger built into the flashgun, and you’re limited in distance due to the power of the light diminishing rapidly over distance. They can be overpowered by ambient light in bright conditions too, such as on a sunny day, making them less reliable when shooting outdoors. Also, your flash has to be in Manual mode, not TTL, because TTL fires a pre-flash before the main flash, which will set off your optically triggered flashgun.
Infrared
Most Speedlights have infrared receivers built-in. Here, a pulse of infrared light is beamed from the transmitter to a receiver in order to trigger the flashgun. They’re superior to optical triggers in that they allow the use of TTL flash due to the fact that the pre-flash won’t trigger them, and they’re more reliable for use outdoors. Also, you can control several flashguns set to different power levels because you can assign them to different groups and channels, which we’ll take a look at in more detail later. However, you’ll still require line-of-sight to trigger them and, again, they only work over short distances – a real pain when you want to hide lights behind walls, light modifiers, or sit them further away from the transmitter.
Radio
This is the system to have above all others. Due to the fact they use radio waves, radio triggers can be fired from much greater distances than optical or infrared triggers. They also work without being in line-of-sight, so you can place your lights behind thick stone walls and still have them fire. They come with the same conveniences as infrared, like groups and channels, but with none of the drawbacks. Traditionally, radio triggers have been much more expensive than other varieties, but nowadays there are plenty of cheaper models that only cost marginally more than infrared triggers.