Object is the parent class of all objects in Unreal. All of the functions in the Object class are accessible everywhere, because everything derives from Object. Object is an abstract base class, in that it doesn't do anything useful. All functionality is provided by subclasses, such as Texture (a texture map), TextBuffer (a chunk of text), and Class (which describes the class of other objects).
Actor (extends Object) An object in three-dimensional virtual space. It is the parent class of all standalone game objects in Unreal. The Actor class contains all of the functionality needed for an actor to move around, interact with other actors, affect the environment, and do other useful game-related things. With UnrealScript, you can write code for any Object class, but 99% of the time, you will be writing code for a class derived from Actor. Most of the useful UnrealScript functionality is game-related and deals with actors.
Owner: Constant Actor declared in the Actor class, specifies who or what owns this object. Use native SetOwner (actor NewOwner); to set the owner.
Pawn (extends Actor) It is the parent class of all creatures and players in Unreal which are capable of high-level AI and player controls.
Class (extends Object) is a special kind of object which describes a class of object. This may seem confusing at first: a class is an object, and a class describes certain objects. But, the concept is sound, and there are many cases where you will deal with Class objects. For example, when you spawn a new actor in UnrealScript, you can specify the new actor's class with a Class object.
Controller: Controllers are non-physical actors that can be attached to a pawn to control its actions. PlayerControllers are used by human players to control pawns, while AIControllers implement the artificial intelligence for the pawns they control. Controllers take control of a pawn using their Possess() method, and relinquish control of the pawn by calling UnPossess().