Steganography
Steganography is the art and science of writing hidden messages in such a way that no one apart from the intended recipient knows of the existence of the message; this is in contrast to cryptography, where the existence of the message itself is not disguised, but the meaning is obscured.
—http://en.wikipedia.org/wiki/Steganography
Steganography differs from overt GPG encryption in that the encrypted message is covertly hidden within innocuous-seeming media files like images, audio and video.
Steganography can be done with text as well as media files, turning a carrier text into a modified stegotext.
http://www.g4tv.com/techtvvault/features/34182/A_Novice_Tries_Steganography.html
Steganography Tools
Steganalysis
Steganographic data can be detected by the process of steganalysis. There are steganalytic sniffers used by corporations and governments that can detect steganographics being transmitted on a network.
StegDetect is a package that is designed to detect steganography.
http://www.landfield.com/isn/mail-archive/2001/Feb/0124.html
Do Terrorists Use Stenanography?
"The USA Today articles were written by veteran foreign correspondent Jack Kelley, who in 2004 was fired in a big scandal, because it appeared that he fabricated a lot of stories and invented sources that didn't exist... A captured al-Qaeda training manual makes no mention of this method of steganography. The chapter on communications in the al-Qaeda manual acknowledges the technical superiority of US security services, and generally advocates low-technology forms of covert communication. The chapter on "codes and ciphers" places considerable emphasis on using invisible inks in traditional paper letters, plus simple ciphers such as simple substitution with nulls; computerized image steganography is not mentioned."
Canary Trapping
A canary trap is a method for exposing an information leak, that involves giving different versions of apparently sensitive information to each of a group of suspects and seeing which version gets leaked. http://en.wikipedia.org/wiki/Canary_trap
—http://www.qwantz.com/archive/001023.html