In man-on-man-sex terms a 'bottom' is the receptive partner in penetrative anal sex, and the 'top' is the penetrator. Because being a bottom is a unique and vastly varying experience, there are further terms that detail some more specific branches of bottom.
Total Bottom: a person who enjoys taking a fully passive and receptive role in bottoming
Power Bottom: a confident bottom who takes a more active and dominant approach to being penetrated
Versatile Bottom: a guy who loves to bottom but is open to occasionally topping
Thumbnail image via Flickr