A specification for data encryption, created by IBM in the early 1970s. DES (Data Encryption Standard) is a symmetric cipher: the very same key is used to encrypt and decrypt the data. It is also a block cipher: it converts fixed length blocks of plain text to ciphertext blocks of the same length.
Theoretically, data encrypted with DES can be decrypted only using the same key that was used to encrypt it. DES relies on 64-bit keys, though 8 bits are used as parity bits for error detection, so the effective key is just 56 bits long. This key length was never considered secure, and in 1998 EFF (Electronic Frontier Foundation) proved that data encrypted with DES could be decrypted in 56 hours.
NIST has since withdrawn the specification, and DES is no longer considered a standard for encryption.