Inverse kinematics (IK) is a positioning and animation method that is built on top of the concepts of hierarchical linking. To understand how IK works you must first understand the principles of hierarchical linking and forward kinematics.
Because of these added constraints, IK requires greater thought about how you link your objects and place pivots. Where many different solutions for linking objects might be suitable for forward kinematics, there are usually just a few good solutions for any given IK approach. The best solution depends on consideration of both the nature of the hierarchy, and how that hierarchy will be animated.