Keypoint is an abstract class. It is the parent of all classes that cause something to happen or have some effect at a particular point in the world.
None (beyond those inherited from Actor (UT), of course).
Keypoints have Advanced -> bStatic = true by default, which makes it impossible to move them by any means during the game, for instance by attaching them to a Mover (UT) (moving them in UnrealEd works fine, of course).
Roughly, Keypoint child classes are blocking actors, sounds, interpolation path points, and special actors that are only useful in single-player maps.