An H-Bridge is a circuit of 4 switches allowing to select the direction of current flowing through a part. The simples H-Bridge consist of four manually controlled switches:
When using a simple H-Bridge it is important to prevent switches to short the supply voltage. In commercially available H-Bridges additional circuits prevent accidental short circuits.
Links
Circuit of an H-Bridge made of transistors, including free wheeling diodes and shortage prevention: http://www.mikrocontroller.net/articles/H-Br%C3%BCcken_%C3%9Cbersicht