Steganography is the science of writing hidden messages, where "hidden" means not only that the message cannot be read by anyone other than the intended recipient, but also that no one else even knows that a message has been sent. Generally a steganographic message will appear to be something else, like a shopping list, an article, a picture, or some other "cover" message.
Steganographic messages are typically first encrypted by some traditional means, and then a cover message is modified in some way to contain the encrypted message. For example, the letter size, spacing, typeface, or other characteristics of a cover message can be manipulated to carry the hidden message; only the recipient (who must know the technique used) can recover the message and then decrypt it. Francis Bacon is known to have used such a technique to hide messages in hand-written letters.
The larger the cover message is (in terms of data content) relative to the hidden message, the easier it is to hide the latter. For this reason, digital pictures (which contain a large amount of data) are commonly used to hide messages on the Internet and on other communication media.
Steganography can be used for digital watermarking, where a message (being simply an identifier) is hidden in an image so that its source can be tracked or verified.