A constraint is what makes everything easier, magic, automatic, customized (add more words here) in a rig. It tells a bone or an object to do something special based on the position of another object, and the position of the constrained object itself. There are many constraint types for you to play with. Most will work everywhere but, the IK solver will only be available in the Armature Editmode or Posemode.
A quick tutorial on Object Constraints used with the physics engine. We should really have called it "Rigid Body Joints". Which is the type of Object Constraint we are demonstrating.