Listed below is a short glossary of terms, should you need it:
A method of concealment in which entire words or phrases are substituted for other words or phrases. Example: dog = hideout. This means that most of what people call "codes" are not really codes, but ciphers (see below). Everything in this instructable is a cipher, except for the book code as noted in step 4. However, since everyone is used to hearing the word "code," I will use it interchangeably with "cipher" throughout this instructable.
A method of concealment in which individual letters are substituted or transposed (switched around). Example: Agent = tnega (Agent backwards). Pig Latin is also a cipher.
A method of encryption in which the letters in the alphabet are replaced directly. This means that everywhere an S appears in your message, it would be substituted with, for instance, M. This is generally a weak type of encryption.
This means that a letter could have more than one meaning. So every time an S appears in your message, the first time it might get replaced with an E, the next time with a W, then a D, and so on. Every cipher in this instructable is poly-alphabetic.
A key is what is needed to decode a message. It may be a word known only to you and your partner (such as the Playfair cipher in step 2), the settings of rotors for the Enigma machine in step 3, or a title of a book for the book code in step 5
Brute force attack
This is when someone tries to break a code by just trying every single possible combination, one at a time, until they get something that makes sense. The average person can break a mono-alphabetic cipher this way, but anything more complicated will likely need a computer.
Your message before it is encrypted. (readable)
The message after it is encrypted (unreadable)