All that is sent is a trigger signal, so there is no TTL/metering information passed, and you are restricted to front-curtain sync up to the camera's X-sync speed or 1/250 second, whichever is slower.

You can use the triggers with any recent-model speedlight that will fire using the centre-pin signal of an ISO-style hotshoe. (You can also fire older, high-trigger-voltage flashes using a "safe sync" adapter, and can use it with the Sony/Minolta flash system or other non-standard flashes using an appropriate hot shoe adapter.)

Trigger-only signalling doesn't necessarily restrict you to fully-manual operation. In a studio-type situation (even an impromptu "studio" set up on location) with a more-or-less static subject, you can get good results with manual flash. (It's a lot easier if the flash has adjustable output. And there are many reliable low-cost manual adjustabe flashes out there, including the popular Yongnuo 460II/Photoflex Starfire and Yongnuo 560 models -- whether David Hobby endorses them or not.) But if your subject -- or the photographer -- is moving around, don't forget that many flashes also have an Auto mode that doesn't meter through the camera's lens. It can be an awful lot easier to work an event or maintain lighting ratios on a larger "stage" if you set the flash for the aperture and ISO you're using and let them figure it out. (It takes a bit of practice to translate what the flash sees to what the camera will see, but it's not all that hard.)

Again, your camera-brand flash may offer an Auto mode. The 580EXII has an auto mode (the 430 does not), as do the Nikon SB-800 and SB-900/910 (but the SB-700 does not). The venerable Vivitar 285, still sold under a number of brands, is also a good auto ("thyristor") mode choice, as are old Metz and Sunpak "potato masher" flashes (which are often available at good prices used). If you want to go well upscale, there is also the Quantum QFlash (which costs about the same per unit as a mid-level brand name monolight).