Compose is messiah's non-linear motion editing system.  Using Compose, you can work with "clips" of motion just like editing video.  It is a non-destructive process, meaning that you are not permanently altering any saved motion; you can go back at any time and restore it back to the way it originally was.

Compose is great for lots of things, but one of the areas it lends itself to the best is television production, where you have libraries of motion for various characters.  For example, you might have two or three walk cycles, two or three run cycles, some jumping or falling animations, etc. and you can quickly assemble those (in Compose) into new scenes.  Or you can do something like blend between two different wing flapping cycles for a dragon.  The possibilities are limited only by the imagination (which means that my possibilities are extremely limited).