"Steganography literally means 'covered writing.' A Greek term, it refers to hiding one message inside another. While the concept has been around for centuries, the marriage of computers and the Internet has brought it to fruition.
Both picture and audio files are ideal for containing hidden messages, he explained; they tend to be very large, and software that substitutes single bits of data at the end of an eight-bit packet creates differences so subtle they can't be detected by the human eye or ear.
In steganography, or stego for short, the visible or audible message is called the carrier, while the hidden message is the payload. Together they form the covert message." More: http://gcn.com/Articles/2005/05/13/Hidden-in-plain-sight.aspx?Page=2
If that isn't enough consider that the hidden text may contain an executable command...