H-Bridge circuits are use to drive a motor both forward and backward. The circuit is called an "H-Bridge" because the arrangement of the switches around a motor form the letter "H".
H-Bridge Circuit Operation
If you connect a 5 volt power supply to a motor you will turn the motor in a specific direction such as clockwise. If you reverse the connections to the motor, the motor will turn the opposite direction such as counter-clockwise.
In order to turn on the motor, switches 1 and 4 must be closed to allow current to flow through the motor. Switches 2 and 3 must be turned off.
To reverse the motor direction you must open switches 1 and 4 and close switches 2 and three in the upper right and lower left portion of the diagram.