A silicon diode is a two-lead semiconductor that gates current flow in one direction. The symbol below shows how a diode is lined up with the schematic symbol. The image was taken from http://www.gadgetjq.com/single_fire_tach_adapter_diode.jpg for copyright purposes. Diodes are used in circuits that convert AC voltages to DC voltages, and also as voltage regulators, clamps, and multipliers.
Current flows in the direction of the arrow. A few other terms of technical use are:
Forward-biased is when the anode is more positive than the cathode, and reverse-bias is the opposite: the anode is made more negative in voltage than the cathode.
There are different kinds of diodes, too. Pin diodes, germanium diodes, schottky diodes, rectifier diodes (p-n junction diodes) name most of them. In this instructable we're going to work with silicon-based pin diodes, although if you're really curious I'd encourage you trying the different kinds of diodes to see how it all shakes out.
Diodes are pretty cheap. You can pick up a pack of 50 from Radio Shack for around $3.