You can consider a character (FBCharacter) as a set of constraints on a hierarchy of models or skeletal joints. This hierarchy is usually composed of instances of bones (FBModelSkeleton), though instances of model root (FBModelRoot) are also used to define a character's reference point and hips.
Generally, a skeleton and weighted mesh (skin) that can receive animation is referred to as a character. The skeleton is used for driving the animation. The mesh (skin) is constrained to the skeleton automatically through the deformer constraints (weighting). You must set up the skeleton and mesh prior to loading the character into MotionBuilder. This is typically done in Maya, 3ds Max, or Softimage.
A character can constrain the data received from an actor, and apply it to a character model to create animation. There can be one or more characters between an actor and a character model. The character can act as a receiver from another character or as a sender. The character can possess a number of sources like an actor or another character, but only one source is active at a time.
Download the create_character_actor_face_etc.py script from http://area.autodesk.com/downloads/scripts/motionbuilder-animation-scripts. Run the script to create an actor, actor face, character, character extension, and character face.