Infographic: A Quick and Comprehensive Guide to Typography
Noodlor offers this free and helpful graphic guide to understanding type.
This section on choosing a typeface based on the message you want to convey is particularly helpful:
Sure, there’s a little bit of the tiresome Comic Sans hating, but all in all, an attractive and helpful thing to keep around, perhaps even print and hang in your office!
A Quick and Comprehensive Type Guide
via NotCot