Centripital force is a force that draws an object toward a center point. Think of gravity acting on a celestial object as it revolves around the sun, or the force that a string exerts on a ball tied at the end of it to keep it from flying away.
Inertia is the forward momentum of an object, the tendency of an object to continue moving in the direction it is already moving in.
Intertia and centripital motion combine to create circular motion, as an object that is moving in some direction away from a center object, but still drawn to that object by a centripital force, will continue to move, at the same distance away from the object, in a circle around it.
