To put it in the simple terms, a Inertial Navigation System is a self contained navigation system. It has a integral inertial unit that gives position, speed, acceleration, etc., but then it also has an integral computer that you can load in waypoints, etc, and it will guide you to those points.
An Inertial Reference Unit only give position, speed, acceleration, but does not have the navigational computer. An IRU must be tied to a FMS system in order to have a navigation system.