What is a screw tap?

A self-tapping screw can be used to join two pieces of plastic or metal with simple holes drilled into them. A "normal" screw requires one of the holes to be "tapped" - a spiral has to be cut into one of the holes to accept the screw. Tapping holes is something of a game of skill; the tapping tools use very hard, brittle steel and it's easy to break a tool in the hole.