The Advanced Encryption Standard (AES) is a specification for the encryption of electronic data established by the U.S. National Institute of Standards and Technology (NIST) in 2001. The algorithm described by AES is a symmetric-key algorithm, meaning the same key is used for both encrypting and decrypting the data.
AES in Bitcoin
AES encryption is used for encrypting the wallet.dat file used in the original bitcoin client. This wallet uses AES-256-CBC to encrypt only the private keys that are held in a wallet.