A somewhat oversimplifed article, but never assume, right. Excellent primer on the basics of credit and debt. http://credit.about.com/od/credit101/a/introtocredit.htm
Five Credit Cards That You Should Never Close http://credit.about.com/od/toughcreditissues/a/closecards.htm has some good advice for when you are on the "recovery" side of credit repair.
Here's Number Six There's a sixth credit card you should never close: the card with your highest credit limit.