The amount of channels decides in how many directions the helicopter can fly:
2-channels: up and down, rotate right and left.
3-channels: up and down, rotate right and left and move forwards and backwards.
4-channels (koax / single blade / avatar): up and down, rotate right and left, move forwards and backwards, and drift from side to side.
6-channels: up and down, rotate right and left, move forwards and backwards, drift from side to side, and upside down.
The best advice for beginners is to start with a 2 or 3 channel model. The few channels are much easier to control. When you are able to fly these (you can find my personal shortlist 2012 here) beginner helicopter without any crash you can move to a 4 channel helicopter. When the 4 channel helicopter is also mastered then and first then move to 6 channel helicopters. Best practice is to train with simulators first. 6 channel helicopters are very hard to take under control and there are many things you have to take care of during flight. And it is a very expensive hobby moving to fast from 2-4 channels to a 6 channel helicopter.