Channels

Can anyone tell me in simple words the meaning and use of channels. What is alpha channel and how is it different from other channels ?

An alpha channel is most commonly used for masking or to save a selection that you want to call on at a later time.

I made tutorial on simple channel masking that will shed a small amount of light on what channels can be used for...(there an uncountable number of situations where playing with channels is called for.)
http://cbutts.com/masking1/index.html

Thanks dear

I will go through it. But one more simple querry, whats the purpose of masking ? Just to hide parts of an image or anything more than that ?