The baseball analogy has been around forever--well, for at least as long as baseball has been. Basically, the metaphor of rounding the bases and heading toward home is used to explain the step-by-step progress people make while heading for a sexual "score." There are no hard and fast rules about what each base signifies (it seems to change with where you live and who you speak with), but in general, each base can be said to stand for the following:

first base = kissing, french kissing second base = "up the shirt," feeling up, extensive fondling, mutual masturbation through clothing third base = "down the pants," feeling down, mutual masturbation without clothes home plate = all the way!, intercourse, oral or anal sex

Hope this clears things up for you!