We don't have the time to send out missing parts by mail, so don't ask. The Cryptography FAQ is posted to the newsgroups sci.crypt, talk.politics.crypto, sci.answers, and news.answers every 21 days. So he replaced every A by a D, every B by a E, and so on through the alphabet. A cryptosystem or cipher system is a method of disguising messages so that only certain people can see through the disguise. Cryptography is the art of creating and using cryptosystems.
Cryptology is the study of both cryptography and cryptanalysis.
The original message is called a plaintext. The disguised message is called a ciphertext. Encryption means any procedure to convert plaintext into ciphertext. Decryption means any procedure to convert ciphertext into plaintext. A cryptosystem is usually a whole collection of algorithms.