An h-bridge motor controller is used for two very useful purposes: one is to control two motors at the same time, the other, is to be able to reverse the direction of the motors at will. This sort of component would come in handy if you were working on anything that you wanted to go back and forth or any two separate directions. A car, a printer, any kind of device that uses a dc motor can be manipulated with an h-bridge. below are two pictures, the first is an actual picture of the h-bridge, the second is a diagram of how to wire it to the arduino.
Okay, to make life easier, here is the diagram in a nutshell:
Here is a code that uses the h-bridge. This is the code from the self driving car fall project listed on the site. Pins axelpmw as well as analog, are the two analog inputs of the h-bridge (1-2en, 3-4en). axel 1-2, ledPin -2 are the motor inputs that go on the two sides of the negative grounds. the code is already pre-commented (if that's a word). good luck and Godspeed.