Cryptography is the science of writing in secret code. It transforms plain text into disguised text and only the intended recipient can decipher it because he knows the key to the cipher.
Cryptography has been used for thousands of years but the computers reinvented it and led to strong encryption.
Caesar cipher (the shift cipher)
Atbash cipher (monoalphabetic substitution cipher)
Polybius square (translates letters into numbers)