Actor: An actor is pretty much any object in UDK that can be placed in a level.
Ambient Sound: Any Sound Cue actor placed to give sound to something in the game world. Generally, these sounds represent what would commonly be called as background noise.
Anim Notify: A function within the Anim Set Viewer that allows us to set up events to be triggered as animation plays. The events that can be triggered vary from Kismet sequences to a sound or particle effect to even events within UnrealScript and others.
Anim Set: As the name implies, this is a set of animations for a skeletal mesh. The Anim Set Viewer offers tools to preview and modify the included animations.
Animation Tree: Animation Trees represent a middle step ...