Questions on rms matching, distortion

Ok, this subject got started in another post. Since i have joined this site i have always noticed most everyone says to match a sub/speakers rms rating to an amp that has a similar rating. Makes sense and sounds good. BUT, in this other post (I'll have to go back and get the link to it) it was said that it would be ok to underpower a 4 sub setup, that by rms ratings would require 6000 watts total, by 4000 watts, thats a BIG underpower issue i would think. So, my question is, which is it?? Ok to underpower or not. Now if it was just a couple hundred watts on a system that large then i could see where it really wouldnt matter but for a more down to  earth system, how much does it matter? What will underpowering a speaker actually do?? Cause distortion?? Which on that note what are some symptoms of distortion?? One other question which is probably related to this is what exactly is "clipping"?? Heard that term alot. Figured these questions would make for some very good knowledge to alot people such as myself. Be interesting to see what kind of replies we get to it.