The Sky's the limit with Photoshop and a tablet pad, but whenever I see an advanced photoshop tutorial (usually for compositing), alphas crop up all over the place! Can anyone give me a few filler points on them as to why and how they are used?(Specifically, why I should use alpha channels for texturing hair in maya, which our teacher showed us last week but I've forgotten it entirely and the one tutorial he offers isn't much help either...See I have to texture a head along these lines in two weeks, but I think know alphas in general will advance me several steps in my knowledge of Photoshop and every other editing package)...
You can have an unlimited about of them, not just RGB, CMYK but as many as you want.
If you have ever saved a selection which is really a mask this will be saved as a channel.
I am sure there is something on the Adobe Exchange that will explain it better. If you have a specific questions, shoot me an email and I'll see if I can help you figure it out. Lance