Steganography

Steganography is an encryption technique in which the existence of concealed data is not obvious to casual inspection. In its simplest form, an image (or some other type of data) is included as an illustration in a (usually Microsoft Word) document. The saved file then appears to be just another document.

In more sophicticated applycations, the data (or picture) can be hidden in an mp3 sound file or in another picture. Play the sound file, it sounds like a sound file. But special software can recover the hidden content. In essence, the data is hidden my making tiny, inaudible imperfections in the file.