Cryptography is the study of the principles and techniques through which information can be transformed from its original form to another that is unreadable to all, except to its addressee, who must have the "secret key" to decrypt the message. It is also a brunch of Mathmatics and part of Cryptology.
There are two kinds of cryptographic keys: symmetric and asymmetric. Through them, one can encrypt the "plain text" (the non-ciphered information). The study of cryptography, however, covers much more than only encryption and decryption. It's a specialized field of information theory, with many contributions from other branches of Mathmatics and knowledge, including authors as Sun Tzu and Machiavelli. Modern cryptography is basically formed by the study of cryptographic algorithms that can be implemented in computers.