For gay men, a "top" is one who is the inserter in anal sex, and the "bottom" is the insertee. This can refer to a specific encounter - "Last night, I topped my boyfriend". Or if somebody exclusively does or prefers one activity over the other, they might refer to themselves as such. "I'm pretty much a bottom." Sometime, gay men who can/do both are called "versatile". Lex
I would also like to point out that there are tops and bottoms for lesbians too. Also, people who can be both.
