Door closers are different than a regular spring or spring hinge in that it closes the door at a controlled speed. Common residential types have pneumatic or hydraulic piston closers that pull the door closed. Most operate with a spring and piston. When the door is opened, the spring is depressed that will activate the door to close. The piston controls the speed that the spring returns back to the original position. Better grades of closers speed up at the last few inches to insure a tight close.