Constraining is a way of increasing the speed and efficiency in which you animate. It's a way of animating one object "via" another one's animation.
In terms of saving time, the value of constraining objects is considerable. Suppose you want to animate a group of people at an air show watching a plane fly overhead. If you used keyframes to animate, you would have to generate function curves for each head movement as well as the plane. With constraints, however, you constrain each spectator's head to the direction of the plane and then animate only the plane itself.
Each type of constraint is described in detail in this chapter except for Constrain 
Curve (Path), which is described in Animating along Paths and Trajectories.
For information on using constraints in character rigs, see Constraints and Rigs [Character Animation].
