He replaced every A by a D, every B by an E, and so on through the alphabet. Only someone who knew the shift by 3 rule could decipher his messages. A cryptosystem or cipher system is a method of disguising messages so that only certain users can see through the disguise. Cryptography is the art of creating and using cryptosystems. Cryptanalysis is the art of breaking cryptosystems breaking the code or seeing through the disguise.
Cryptology is the study of both cryptography and cryptanalysis.
An original message is called plain text. The disguised message is called cipher text. Encryption refers to any procedure to convert plain text into cipher text. Decryption refers to any procedure to convert cipher text into plain text. The intended individual who is supposed to be able to see through the disguise is called a recipient.