ROLE-PLAYING: WHAT IS TOP AND WHAT IS BOTTOM

In BDSM, a TOP is a partner who takes the role of giver in such acts as bondage, flogging, humiliation, or servitude.
The top performs acts such as these upon the BOTTOM, who is the person receiving for the duration of a scene.

Although it is easy to assume that a top is dominant and a bottom is submissive, it is not neccessarily so.

The top is sometimes the partner who is following instructions, he tops when, and in the manner, requested by the bottom.
A person who applies sensation or control to a bottom, but does so to the bottom's explicit instruction is a service top.
Contrast the service top with the pure dominant, who might give orders to a submissive, or otherwise employ physical or psychological techniques of control, but might instruct the submissive to perform the act on him or her.
The same goes for bottoms and submissives.
At one end of the continuum is a submissive who enjoys taking orders from a dominant but does not receive any physical stimulation.
At the other is a bottom who enjoys the intense physical and psychological stimulation but does not submit to the person delivering them.
It should be noted that the bottom is most often the partner who is giving instructions—the top typically tops when, and in the manner, requested by the bottom.

Original still from TALKING ABOUT RUBBER BOOTS AND MACHINE GUNS