Everyone defines the "bases" differently, but here are 2 of the most common:
1st Base - Kissing. (Kissing with tongues, also known as French Kissing.)
2nd Base - Touching. (Girl touches/gives hand job to penis, boy fingers or touches vagina and/or breasts.)
3rd Base - Oral sex. (Girl sucks boy's penis, boy eats out [licking] girl's vagina.)
4th Base (Home Run) - Sexual intercourse. (Penis in vagina.)
Another account is:
1. Kissing
2. Shirts off
3. Pants off
4. Sex