To pose a character with forward kinematics, you rotate each joint individually until you get the desired positioning. For example, to move a hand to some location, you must rotate several arm joints to reach the location.
When you animate a skeleton posed with forward kinematics, Maya interpolates the joint rotations starting with the root joint, then the root’s child joints, and so on down through the skeleton’s action hierarchy. Maya proceeds forward through the action hierarchy, starting at the root joint.
Forward kinematics is intuitive for creating simple arc motions, but it’s tedious if you are animating a complex skeleton. It’s also not intuitive for specifying goal-directed motion. For example, to move a hand to some location, it’s not obvious how to rotate the joints in an arm.
With IK, you create an extra control structure, an IK handle, for certain joint chains such as arms and legs. An IK handle lets you pose and animate an entire joint chain by moving a single manipulator.
As you pose the IK handle, it automatically rotates all the joints in the joint chain. For example, if you move a hand to a doorknob, the other joints in the arm rotate to accommodate the hand’s new positioning.