Ok guys, I'm sure this has been asked somewhere else before, but darned if I could find it thru Google. What's the difference between the various types?

Oh yeah, one more quick question - I'm eyeing an Armalite m15-a2 for around $600 - is that a decent price for them? I seem to remember Armalite running around $800-$1000 back when I first got interested in Ar's.

Thanks in advance for your help!