Anything that moves, changes, or can be affected by other actors.
Actors have a ton of properties which can be accessed by right clicking on the actor.
Brush - A brush is the basic unit used to construct levels. Generally a brush is a simple three dimensional shape that can be added to or subtracted from the world to form more complex shapes and structures. UnrealEd can build several types of brushes, called primitives automatically. Be sure not to confuse a brush that has been added or subtracted with the building brush . Building Brush - The building brush is the red wireframe brush you see when you build a brush.