Steganography is a technique used to hide messages in plain sight. This differs from cryptography because most of the time you won't even realise that a message has been hidden. This technique is commonly used to conceal information inside files that appear to be normal, such as inside images.
This can start off very simple such as having a plain coloured background for an image, then changing the colour slightly and entering a message into the image or go as advanced as using programs to encrypt the message and embed it into the image. This however is very hard to decrypt without know the original encryption used.
It can also be used to hide messages inside a paragraph, for example:
Taking the first letter of each word, or simply:
Taking each capital letter. This technique is also know as the null cipher.
It is also very easy to hide messages within a sound (such as an mp3) file. The most commonly used method is a method known as backmasking in which the message is reversed and added to the sound file. Also speeding up the message by a large amount so that it just sounds like static is also effective.
Then there's hiding messages in videos, this is very easy to do because of the high frame rate videos use. For example animation can use from 10 to 30 frames a second, meaning if one of those frames is altered slightly chances are no one will notice. Words could be added to one of the frames or the frame could be changed to display a hidden message within certain keyframes.
Steganography has been used throughout history in a variety of ways and has grown recently with the digital age, for more information check out the Wikipedia article on the subject.