From Wikipedia:
"A backward message (also known as Backmasking) is a supposed message hidden in an audio recording that is only fully apparent when played backwards."
Of course, the classic back-masked message is the one in Stairway to Heaven, but my personal favourite back masked messages are all deliberate:
Pink Floyd - Empty Spaces: "...congratulations. You've just discovered the secret message. Please send your answer to Old Pink, care of the funny farm, Chalfont,"
Wierd Al Yankovic - Nature Trail to Hell: "Satan eats Cheez Whiz"
Sisters of Mercy - Vision Thing (Apocalypse now quote): "Only the Americans would build a place like this in middle of the jungle. Only the Americans would want to."
Also, though it's not really back-masking (because it's played forwards) the KMFDM classic 'Sucks', after the lines "KMFDM forwards the ultimate sound, with a message from Satan when you turn it around" has an answering machine message from Satan to Sascha, asking him if he wants to play pool and have some beers sometime.
Also anecdotally, I heard somewhere that Motley Crue once had to delay putting an album out becuse their sound tech quit when they asked him to do some back-masking.
So, bring out your messages, Satanic* or otherwise, real or imagined.
*The only actual Satanic message I know of in any song is in 'As Flittermice As Satans Spys' by Darkthrone, which has a reversed recording of Varg Vikernes saying 'In the name of God, let the churches burn'.