Blender 3D: Noob to Pro/Advanced Tutorials/Advanced Animation/Guided tour/Const/cr
This constraint copies the rotation of the target. As simple as that. It can be an object or a bone. As you can see in the example, only the rotation gets copied.
The Constraint Panel↑Jump back a section
Where To Use It
Can be used with Copy Location to fake parent relationship. As you can key the influence you can make a character pick something up and hold it in his hands. Check the .blend for the hand-glass scene.
(?)link to the Blend(?)
You can also use this to align a plane with a 2D effect on it to the camera at all times. This works better than pointing it at the camera in some cases, such as a ring of atmospheric halo around a planet, where you don't want it disappear behind the planet.