Inverse kinematics is just like kinematics, but backwards. Given a linked, constrained structure, IK allows one piece of the structure to be positioned, and the rest of the structure to automatically position itself to fit the constraints. So, given an IK model of an arm, pulling a finger forward will cause the hand to move, which will cause the elbow to move, which will cause the shoulder to pivot. At the end of this, the arm is positioned naturaly, with the finger pointing outwards and the rest of the arm supporting it. IK is a feature of many high-end 3d modeling programs.