what is the difference between a2 channel plane and a 4 chanel plane?

The best answer is by :
VinceHerman :
Often, a 2 channel plane has 2 motors, and it steers by making one motor go faster, and one go slower. There are no moving control surfaces like a rudder or elevator.
Planes like this are just barely controllable. And very difficult to land, since you have to shut down the motors to land, and the motors are the only control.
The good thing is that they are very inexpensive. The bad thing is that they really are not much fun, and many who try them give up on the RC hobby.

A 4 channel plane probably has a throttle for the motor, a rudder to help turn, an elevator to make it fly up or down, and maybe ailerons to make it bank.
These are much better than 2 channel, differential thrust 2 channel planes.

Before you buy, go read up on lots of beginner RC plane issues on the dedicated RC palne forums:
http://www.rcgroups.com/beginner-training-area-electric-8/
http://www.rcuniverse.com/forum/forumid_85/tt.htm

Good luck!