From RoboBlitz Editor Wiki
Object is the base class of all classes in UE3. It contains math functions and various other helper tools.
Actor is the highest level class that represents something in the game world. It is a very extensive class with lots of functionality.
RDestructible is the primary base class for everything in RoboBlitz that isn't nailed down (the things that are nailed down, the floor, walls, or buttons are static meshes or BSP). (Note: Karl may be nailed to the ceiling, but he's actually an RDestructible ;) )
Projectiles are the Unreal Class which impliments projectiles in the game world.
RProjectile is a RoboBlitz specific implimentation of Unreal projectiles. All projectiles in your mods should inherit from RProjectile.
You can see a full listing of classes available to the RoboBlitz Editor by opening the Browser Window to the Actor Classes and expanding classes. Each class in the browser contains all classes which inherit functionality from it. Expand each section to view the children classes. Many of the interesting classes inherit from RDestructible