The scientific name for the art of hiding messages is steganography. In one form or another it has been practiced since ancient times. With the development of computers it has become possible to hide a message (the contents of a computer file) in another file so that the addition of the message to the container file will not noticeably change the appearance of that file. If the file which has been hidden is deleted then there is no indication that it even exists, but it can still be recovered when you need to get at it.

http://stego.tribe.net