Answer:
In some ways, 1st base can mean boyfriend girlfriend, 2nd base can mean kissing, 3rd base can mean making out, and home run can mean... well.... you know.
------------------------------- *EDIT* ----------------------------------
I think the most common version is:
1st = Kissing
2nd = Touching (Cloths Fully On)
3rd = Touching (Under Cloths, Below)
4th (Home Run) = You Can Guess