Topping and bottoming
Topping is the doing side of most BDSM activities. It is the tying of the knots on the rope which restrains your partner, the wielding of the flogger, the locking of the cage, the applying of the ice, and the dripping of the wax on your partner. Some people also call this the "sending" side of BDSM.
Topping relates specifically and only to the activities which you might find in a BDSM scene.
Bottoming also relates just to BDSM scene activities. It is being on the receiving end of what happens. It is being hit with a flogger, being tied up, having wax dripped on your nipples, having ice applied to body parts which don't normally see anything that cold, having designs cut into your flesh, etc.