To add an aside to that - Poser's concept of a bone is really nothing more than a means of picturing the joint centres and the hierarchical relationships when using the setup room. I don't recall the term being used before Pro Pack (the first version with a setup room included) was released. As such, it only goes part of the way way towards describing what happens when a joint bends. As nruddock says, the rest of the important stuff is covered by the concept of the actor.

I understand that other 3D application's rigs have a more comprehensive skeleton, but I know nothing about them so should probably avoid saying any more. ;)