Plaintext: Everyday language; e.g. "This is plaintext"
Ciphertext: Enciphered language; e.g. "rgua ua xuogwerwzr"
A code is a system of secret communication in which each word in a message is replaced with another word, letter, sentence or symbol. For example:
|Plaintext:||British||won (the)||battle (at Alam Halfa)|
There are two parts to every cipher, and in order to decipher messages you must know both parts:
Cipher = Algorithm + Key,
where an "algorithm" is a general method of encryption, for example, "swap every letter for a symbol", or "jumble up all the letters in a wird", and a "key" is the particular way a message has been enciphered that time, for example, "replace A by $, B by &", or "move the last letter of every word to the front".