"When a message is encrypted, its contents are more or less jumbled in order to baffle anyone who might intercept it. But if something is '
stegoed,' it is secreted out of sight, like invisible ink. No one but the sender and recipient know it's there, making its discovery much less likely. When used together, the two techniques, encryption and steganography, pose a double whammy for code-breakers.
With the Internet, steganography has come of age. Stego-tools free and easily downloadable software programs take advantage of the unused space in audio, text, image, or video files and create a secret hiding place for embedding data. Take an image of Osama bin Laden, for example. Then take a map. A stego-tool can be used to hide the map file under the 'cover image' (stego-speak for the original file). You have the digital age equivalent of invisible ink."
Catherine Auer, "Behind the bits; steganography used for subversive activities," Bulletin of the Atomic Scientists, May 1, 2001