In photography, when you're trying to add flash and want it 'off-camera' there's a few options. Off-camera flash is important, because let's be honest, on-camera flash looks harsh and just plain bad. So we have to move the flash away from the camera. Sometimes it's to the right, sometimes left, sometimes up. Other times you might add more than one flash to light the frame.
One way is to use a sync cable. This involves the possibility of adapters, and stretching a long cable between you and the flash. My camera doesn't have a sync socket, so I'd need an adapter.
Another way is to use an optical flash. This is a flash that is triggered any time it sees any other flash light up the scene. I have one of these, and it's great until there's other people in the same area with flash cameras. Then it gets flashed for them instead of me.
The third way is through a radio remote. The most popular of these is called the PocketWizard, and sells for around $350 for a pair (transmitter + receiver.) I didn't have that kind of cash lying around so I ordered a $40 (including shipping) knock-off that people lovingly call a 'PovertyWizard.' Mine came in the mail today.
Rough testing showed I get about 20 feet range with them. I ordered mine planning to disassemble, violate some FCC rules and add an antenna to the transmitter. People have reported upwards of 100 yards after doing that modification. I also plan to modify the receiver to use standard AA batteries instead of the wacky 3 volt that it comes with.
Here's my first attempt. The addition of the radio transmitter lets me trigger my old Pentax flash by radio, and use the light from it to trigger my Digi Slave 3000 optically.
The Pentax flash is behind and above her as a highlight. The optical slave flash is above and right in a bounce umbrella. I'm not really happy with the shot, but hey, it's my first picture ever taken with two flashes, neither of which is attached to the camera.