alternate reality game
An ARG is an interactive story that uses real-world media as a platform.
As the story is told in real time, events can be (and are often) influenced by
the players actions.
The story is typically advanced through puzzles that the players must solve. These puzzles can be rather complex, more so than those found in a traditional game, as they have the benefit of many players working together.
hiding information
Many puzzles feature some sort of encoding or steganography, where the goal of the puzzle is to uncover hidden information that leads to the next puzzle.
As the game progresses the puzzle tend to become more difficult (or they may require more obscure knowledge).
simple ciphers
Some of the most well known ciphers are substitution ciphers; Each letter in the plain text is transformed into another by some reversable method.
steganography
Any encrypted message is a fairly obvious sight, but it is possible to to hide a message within another medium. This has the advantage of making even the existence of the message unknown to those who are not looking for it.