+1, the wiki is a great resource (or was--does the link not work anymore?). So also found another good one at EngadgetHD if you wanna get your learn on:

http://www.engadgethd.com/2009/02/19/hd-101-why-there-are-black-bars-on-hdtvs/

Wierd thing...I just added a note to my original post.....when copy/pasting a hyperlink, for some reason, the ending parenthesis is not included. That is why the link was not working. It does the same if you post it into Microsoft Word. If I had the time/energy, I would try to figure out why.

Anyway, try this:

After clicking the link, add the ending parenthesis at the end of the address and hit enter. Or just copy and paste the link into your browser address bar. Sorry for the confusion...I must have looked kinda silly, eh?

http://en.wikipedia.org/wiki/Aspect_ratio_(image)