Diode
From uCtrl.net
Information
A diode is a semi-conductor that only allows current to flow one way, positive and negative will flow in opposite directions. This is the principle behind the rectifier, see image below. Diodes can be used for security; such as stopping reversed polarity or for stopping current to leak where one does not want it. OR-gates can also easily be constructed using diodes.
For more information on diodes as relay protection, read the relay article.
Rectifier
