"Top" and "bottom" are used informally to refer to the inserting partner (the Top) and the receiving partner (the Bottom). Trouble is, most of us perform both roles at one time or another in a relationship. And some of us "69" -- simultaneously give each other oral sex -- so we're being a Top and a Bottom at the same time.
Some people have also used the words to refer to the more masculine-looking partner in a relationship as the Top and the more feminine partner the Bottom. But of course how you look has nothing to do with what you do in the bedroom!