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. It's natural to say that n is the key here. The people who are supposed to be able to see through the disguise are called recipients. Other people are enemies, opponents, interlopers, eavesdroppers, or third parties. For an introduction to technical matter, the survey articles given in part 10 are the best place to begin as they are, in general, concise, authored by competent people, and well written.