In general, people refer to French kissing — open mouth kissing — as first base. Second base is usually manually stimulating — using one's hand on — a woman's breasts. Third base is manual stimulation of the genitals — guys' or girls'. And hitting a homerun generally refers to having vaginal intercourse.

The problem with these very old-fashioned metaphors for sex play is that they imply that there is a competition going on between sex partners. Today, we are more aware that sex partners are equals, and that competing against one another to get what they want sexually doesn't make for healthy relationships. We know that sex should be about communication, not about competition.

Hope this information helps!