Backtracking, or backmasking is the form of inserting hidden messages into tracks which can only be consciously heard when played backwards. Most of the time these messages are inserted purposefully, sometimes by accident, some people even believe that the subconscious can actually pick up these messages. But the nature of the backtracked messages in many modern pop songs is so sinister as to leave no doubt to the intentions and affiliations of these singers. 
http://www3.telus.net/jefmil/stairwaybackwards.htm
link to flash file:
http://www3.telus.net/jefmil/backmasking.swf
The britney spears one is hilarious, and the Led zeppelin one is pretty cool too. Some are pretty far fetched though....