Try using it in a sentence.

On what context you are using the words.
In the BDSM world, top=dominant (ie: doing things TO someone), bottom=submissive (having things DONE to them).

In the homosexual world, top = guy who is on top, ie.. pitching? ie: penetrating... bottom = guy who is on the bottom, ie... catching, ie: being penetrated.

Hope that helps!
Hugs*
Nicole

those are terms most often used by gay men.  Pretty self explanatory, actually.  If you are the recepient of the cock, then you are the bottom...if you are the giver of the cock, then you are the top...if you are versatile, then you do both...I guess some are entirely bottoms and some are entirely tops, but I guess there are still BJ's given for the bottom's satisfaction.