Alright, so steganography isn't really a form of cryptography, it's just related. However, I still think it belongs in this series because it is often paired with cryptography... and because it's really interesting.
But what is it?
Steganography is the art of hiding things, specifically messages. It is such a crucial and basic art that it may have been invented long before cryptography. The name comes from Greek, specifically the word steganos, which means "covered or concealed". Steganography, is more than just hiding things, it's hiding them in plain sight.
First Usage
As an interesting side-note, the first book about steganography was called the "Steganographia". But it wasn't really about steganography, or at least it didn't seem to be. On the surface the book appears to be about the magical arts, and communicating via spirits. It must first be decoded using hints sprinkled throughout it before the true message is revealed. And thus, the first book about steganography is an example of it itself.
History
The Steganographia isn't the only interesting usage of steganography. It was reported to have been used throughout history, during many influential periods.
One of the simplest examples of how steganography was used in the past is the "wax tablet technique". Back in the days of the Greeks, most people, when they wanted to write something, would carve it into a wax tablet. At some point someone had the ingeneous idea to scrape off the wax from the tablet and carve a message into the wooden base. The tablet could then be re-coated with wax. In this way, secret messages could be passed through an ordinary implement, and no one would be the wiser unless they scraped every tablet they came across.
Another famous example of steganography comes from a Greek named Herodotus, the so called father of history. Herodotus describes a secret message being passed to the Greeks to preemptively warn them of Persian invasion. The message was tattooed onto the shaved head of a servant. The servant's hair was then allowed to regrow, thereby hiding the warning message. In this way the servant was able to travel to Greece with out raising any suspicion. When he arrived, they simpy shaved his head and read the tattooed message. This obviously isn't one of the fastest methods of steganography, but supposedly it worked.
Nowadays
All the above methods seem very primitive, but steganography is still in use today. One of the most obvious examples is the use of invisible ink. Because invisible ink temporarily hides a message it is a type steganography. In modern times steganography has expanded to include hiding information within digital files like images.
Hopefully you found this interesting, and if you did you'll probably enjoy the wikipedia article on steganography. There's much more to learn!