There are several types of self closing door hinges. These units are small and very concealable by your doors frame, as they generally appear as a simple hinge that slips into your doors frame. These have removed the need for the bigger bulkier hydraulics arm units used for so many years in the past.
Spring Loaded Hinges
One specific model of self closing door hinges, uses the friction caused by opening the door to pull a spring. This spring uncoils as the door is opened, and when you let go, the spring closes the door by pulling back to its natural state. The spring loaded hinges work well, however can break or snap with age, causing them to potential damage your door frame or bystanders.
The more expensive type of self closing door hinges use hydraulic pressure to close the door. These are much more expensive than the spring loaded versions and may have to be mechanically adjusted for speed. These types of hinges work on a vacuum system, so that when you open the door, a plunger sucks air out of a chamber, the loss of this air creates a vacuum. When you let go of the door, the vacuum pulls the plunger back into the empty space, closing the door.