The encryption mechanism uses a value known as an encryption key to encrypt it. Similarly, a decryption mechanism uses a decryption key. It is this key length that decides how strong or secure the particular encryption is. Thus, a cryptosystem 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.
Cryptography has very ancient beginnings. There is a story that When Julius Caesar sent messages to his trusted acquaintances, he didn't trust the messengers. So he replaced every A by a D, every B by an E, and so on through the alphabet.