It's not the best, but you can google delta fare class chart, and one of the first results is a table sort of explaining them all.

http://cwsi.net/delta.htm

The biggest differences in fares have to do with upgradeability, MQM earning, and refundability. Delta.com is really the best source for upgrade rules on fares, MQM earnings, etc. but you might have to do some searching to get to the right page.