Zbrush is considered 2.5D, if I'm not mistaken. You use it similar to a sculpt program to create extremely high detail, super high res models. I'm talking millions of polys. I would doubt you would ever animate a zbrush model, although I'm sure it's possible.

The biggest use for it that I can see, aside from making super high res models for stills, is to bake the surface information into displacement maps or normal maps (for games).