Basicly its simplified model that you use to build\modify more complex one. It can be used for making mechanisms also.Skeleton parts dont show in drawings or have mass props as they are just used to define the model. Skeleton is really a nice name for it. Basicly if you would to build human body in Creo you would build skeleon first and attach evrything to it. Changin lets say lenght of one bone would change size of entire ligament etc.
Look at these 2 videos:
http://learningexchange.ptc.com/tutorial/2291/intro-to-top-down-design-skeleton-models
http://learningexchange.ptc.com/tutorial/3261/motion-skeletons
Cheers