You have a number of options:
1. Connect the slaves to your camera with a cable. Buy the cable from Canon. I do not recomend this because you will have cables snaking all over the place.
2. Sync to the slaves via IR (ETTL function). If you own a 550EX, 580EX, or ST-E2 transmitter you can fire your slaves using IR. This is the best option but involves you investing in one of these flash heads. To maximize the IR range, you need to turn the body of the flash so that the IR reciever (sits under the red transparent plastic panel) points at the main flash, but aim the flash head at the subject.
3. Fire the flash using an optical trigger. You can buy a cheap sensor which detects another flash firing, and that triggers off the flash it is connected to. Only advantage is that you can place the optical trigger anywhere you want. Disadvantage: the flash must be fired in full manual mode and you lose the ETTL function.
4. Fire the flash using a radio trigger. These are expensive ($300 per transciever!). You lose ETTL so you must control the power output manually. Main advantage is that you can fire the flash from massive ranges and do not need line of sight to fire the slaves.
What is channel A, B, C, D?
This allows you to group your slave flashes and set different power outputs for each. This is set on your master commander flash. Example:
550EX flash (attached to camera): Channel A
420EX flash (placed in a dark corner of a room): Channel B
Another 420EX flash (placed in another dark corner of the room): Channel B
Now, I will not want my 550EX on the camera to fire at maximum power because it will overwhelm the subjects. On the other hand, the 420EX'es need to fire at full power because its job is to illuminate the dark background. If I put the 420EX on the same channel as the 550EX, both will fire equally. So I set the A:B ratio to (say) 1:4 - meaning channel B flashes fires at 4 times the power of the main flash on the camera.
