DES was, for many years, the encryption standard used by the U.S. government for protecting sensitive, but not classified, data.  The American National Standards Institute (ANSI) and the Internet Engineering Task Force (IETF) have also incorporated DES into security standards.  DES is by far the most popular secrete key algorithm in use today.

The original standard of DES uses a 40-bit (for export) or 56-bit key for encrypting data.  The latest standard, referred to as triple-DES (3DES), encrypts the plain text three times using two or three different 56-bit keys.  This approach produces ciphertext that is scrambled to the equivalent of a 112-bit or 168-bit key, while still maintaining backward compatibility.

DES is designed so that even if someone knows some of the plain-text data and the corresponding ciphertext, they have no way to determine the key without trying all possible keys.  The strength of DES encryption-based security rests on the size of the key and on properly protecting the key.  Although the original DES standard has been broken in brute force attacks of only 56 hours, the new 3DES standard should remain secure for many years to come.

(Source:  "Security Complete" 2nd Edition, Sybex")