The four bases in a relationship, like in baseball, where the partners try to get home. First base is equivilent to french kissing, not just kissing. Second base is touching of private extremities and/or apendages of the partners' body, aka boob touch. Third base is oral sex. Lastly a home run or four bases is full on, big time sex.
Person A: So how did your date go last night?
Person B: Pretty good. Me and Susie got to first base!
Person A: I didn't know you knew what the bases of dating were. Haven't you two only been dating for a few days?
Person B: Yeah, I'm going to try and see a movie at her house while her parents are out next week, mabye we can get to second!