A cipher is a way to make a word or message secret by changing or rearranging the letters in the message. (A code affects the word, not the individual letters.)
One type of cipher, called a TRANSPOSITION CIPHER, is created by simply rearranging the letters in the word itself. For example, CHYPRAGTOPRY can be unscrambled to reveal the word CRYPTOGRAPHY. Another cipher, the SUBSTITUTION CIPHER, is a bit harder. It involves changing the letters of your message into something else: other letters, numbers, or symbols. Using a substitution cipher, the word CRYPTOGRAPHY might look like this: DOHQMRZOFQYH. In this example C=D, R=O, Y=H, and so on.
For your friends to understand the message, they must know your system. In our substitution cipher, your friend would need to know the KEY, or how the system is set up (a=f, etc.). If only you and your friend have the key, then it is very difficult for others to read your message.