NoLuck, the diode basically allows current flow in only one direction. It acts as a one way electrical "valve". Some of the main applications of them are to change AC voltage into DC voltage (called retifying) and to prevent electical feedback spikes from solenoids and relays from damaging the electronics. You place it in parallel with the coil of a relay or solenoid and it filters out the high voltage spikes when the coils are de-energized. I also like to use them to prevent accidental reverse polarity connection of my battery supply. I cant tell you how many times I have accidentally hooked the positive lead of my battery supply to the ground of my boards.
The diode has a band on one end of it. This is basically showing you want end the current flows to. current will flow only if the end without the band is more positive than the end with the band. If you put a +5 volts on the end with the band and ground on the other end, then no current flows. Hope this helps.