Bridges are used for full wave rectification. If a sine wave is put across a diode and a bridge, the diode will rectify half of the wave, while a bridge rectifies all of it. The diagram below shows the output wave forms of a diode and bridge when they rectify a sine wave.
Bridges are composed of four diodes. The diodes are arranged so that there are two inputs for an alternating current, and a positive and negative output. The first schematic of a bridge above shows how it is made.
A picture of some bridges is shown above. Depending on the current carrying capacity, some bridges have heat sinks, such as the first two on the left.