Wikipedia: Cryptography

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Cryptography (from Greek kryptós, "hidden", and gráphein, "to write") is the study of the principles and techniques by which information can be concealed in ciphers and later revealed to legitimate users employing the encryption key (see /Key), but in which it is either impossible or computationally infeasible for an unauthorized person to do so.

More specifically, cryptography has four main goals:

message confidentiality: It should only be possible for those authorised to read a message to do so. It should be impossible (or at least extremely difficult) for unauthorised persons to obtain the message contents.
message integrity: The receiver of a message should be able to verify whether the message has been altered during transmission.
authentication: The receiver of a message should be able to identify the sender and verify that they did transmit the message.
non-repudiation: The sender should not be able to deny sending the message.

: Cryptology -- Cryptanalysis

: Hash function: MD5 -- SHA-1
: Public Key Cryptosystems (Asymmetric Algorithms): RSA -- Diffie-Hellman -- Elliptic curve cryptography
: Secret Key Cryptosystems (Symmetric Algorithms): Enigma -- One-time pad -- Data Encryption Standard (DES) -- RC4--Blowfish -- International Data Encryption Algorithm (IDEA) -- Advanced Encryption Standard

: Terminology: /Key -- Cipher -- Code