Back in the days of the dinosaurs when I grew up, I learned the following:

First Base - holding hands; kissing (no tongues)
Second Base - touching, through the clothes; french kissing
Third base - touching/fingering under the clothes; clothes off (barely) / skin-to-skin
Home - all-the-way

Nowadays [2009], Urban Dictionary informs me it’s:

First Base - French kissing
Second - fingering/hand-jobs
Third - Oral sex (and anal if desperate)
Home - ... [My thoughts:  If you’re having anal sex with someone and still using baseball analogies, grow up, sheesh.]