From BlenderWiki

The Stack

As said above, constraints are evaluated from top to bottom of the constraint stack, shown/materialized in the Constraints panel.

In (A constraint stack example), first the location of the lamp is copied to the owner object (Cube, as shown by the To Object static text field). The copy rotation constraint is ignored (red name, see below). So the next constraint evaluated is the Child Of one, which is currently reduced. Finally, the size of our cube is bounded by the Limit Scale last constraint. So here, the size of the cube is first controlled by the target of the Child Of constraint, within the limits allowed by the next Limit Scale constraint… As with modifiers, order is crucial!

You can move a constraint up and down the stack by using the small up/down arrow buttons that are drawn in its header, to the right of the constraint name. These buttons are only visible when needed, i.e. the top constraint has only the “down” button, the bottom constraint, only the “up” one – and when there is only one constraint in the stack, both buttons are hidden.

Adding/Removing a Constraint

To add a constraint, you can, in the Constraints panel, click on the… Add Constraint button! A menu shows up, listing all available constraints for current active object (or bone in Pose mode). The new constraint is always added at the bottom of the stack. Right to this control, you’ll find a To Object (or To Bone in Pose mode while rigging) static text field, that shows the name of the “owner”, i.e. the object/bone being constrained.

You can also, in a 3D view, either:

• Select only the future owner, hit CtrlAltC, and in the Add Constraint to New Empty Object menu that pops-up, select the constraint you want to add. If the chosen constraint needs it, a new Empty object will be automatically added as target, positioned at the owner’s center, and with null rotation.
• Select first, the future target, and then the future owner, hit CtrlAltC, and in the Add Constraint to Active Object (or Add Constraint to Active Bone) menu that pops-up, select the constraint you want to add. If the chosen constraint needs it, the other selected object/bone will be used as target.

Note that these pop-up menus do not propose you all the available constraints…

To remove a constraint, click on the “X” button of the header of the constraint you want to delete, in the Constraints panel. You can also remove all constraints from the selected object(s), using the Object » Constraints » Clear Constraints (or Pose » Constraints » Clear Constraints… or hit AltC, in Pose mode), and confirm the Clear Constraints dialog that pops-up.