This webpage, SmashMagazine.com - Color Theory for Designers (Part 1) is a great way to become familiar with how color affects composition, mood and overall design.
Here's Part 2 and Part 3.
Overall, Smashing Magazine looks to be a good overall source of info for those of us who can get lost and confused among all the details.
If ya need help in choosing colors or color scheme, Adobe's Kuler is awesome. The link: kuler.adobe.com