Acronym for Data Encryption Standard
A very widely used and widely studied encryption algorithm. It was designed by a team at IBM under guidance from the NSA and standardized by the NIST. There are a number of variants, including 3DES. Its designated successor is AES.
An NSA recommendation that DES implementations shouldn't use certain parts of the keyspace led to a lot of controversy about whether a backdoor/flaw that allowed NSA to break DES encrypted messages existed. A new attack on ciphers found after DES being in use for about 30 years was successful against almost all ciphers at the time, including DES for that the part of the keyspace. Many believe that NSA knew about this attack (30 years before everyone else!) and "fixed" DES to be safe against it. Take this little story with a grain of salt and use your tin foil hat.