UE2:Actor

From Unreal Wiki, The Unreal Engine Documentation Site
UE2 Object >> Actor
Known custom subclasses:
Round_Robin, PawnFactory, IncrementalTrigger
This class in other games:

The (usually abstract) base class for about anything that is visible or can be interacted with in a level.

See the various game-specific classes for details. Subclasses linked above are mostly articles about generic custom classes for Unreal Engine 2 games.