Here's a video talking about the basics of pose space deformation. This is definitely one of those things that one should be aware as a rigger. The basic idea of pose space deformations is to create modeling fixes for certain areas based on the poses that that area will hit (a lot of times you can use the term "corrective blend shapes" for this type of thing as well). The math/programming behind creating pose space deformations can get tricky, but fortunately there are a few resources out there that will handle it (I use BSpiritCorrectiveShape in the video). The gist is that you pose your already rigged character and then make changes based on the deformations, etc IN THAT POSE. The tool you use should remove the influence of the joints, etc to calculate what the actual transformations of the vertices (or CV's) are in local space and give you a corrected model that can be worked in as blend shape or what have you. Really a great way to do two things: a) fix a model that's not deforming correctly based on weighting alone and/or b) add some movement specific deformation to enhance the rigging. Anyhoo, here's the vid:
Maya/Rigging: Pose Space Deformation from zeth willie on Vimeo.