Wireless systems are triggered in one of two ways: optically by either infrared light or the visible light from the master flash unit or via radio signal from the transmitter to the receiver(s). In both cases there are systems that are capable of TTL when the various camera/flash combinations are all compatible with a single manufacturers system, and there are other systems that are only capable of triggering flashes set using manual controls. A Pocket Wizard trigger capable of using Nikon's i-TTL, for example, will not be able to control TTL functions for a flash made for Canon's E-TTL system, even if the receiver is on the same radio frequency. It should be able to trigger the flash to fire using manual controls on the flash.
If a flash is capable of being triggered by an optical flash of bright light, it should fire regardless of the source of the light. The limiting factor is the brightness of the source light in certain environments. When working in bright sunlight the range of the master unit will be less than in a less bright environment. Optical triggering can also be problematic if you are trying to photograph an event and your flashes are triggered every time someone takes a picture with their camera phone.
Radio triggers allow different photographers to use different frequencies so that each can discreetly trigger only their own system. Some triggers have the radio frequency they use labeled on the transmitter and receiver(s). If all you are going for is manual control, then any trigger/receiver combo should work if they are on the same frequency unless the signal to "fire" is encoded within that frequency.
In practice, if all you want is a way to tell a manually controlled flash to fire, start out with a cheap set of triggers like the Cowboy Studio NPT-04 plus extra receiver. If you find the range is too short for your needs or the durability is not up to what you will put them through, you're only out about $30 and you will have a better idea of what additional features are important to the way you want to use them.