Acronym for Data Encryption Standard
A very widely used and widely studied encryption algorithm, now being gradually replaced by AES. There are a number of variants, including 3DES.
The workings of the algorithm are explained: 
http://www.tropsoft.com/strongenc/des.htm and the official page at the NIST: http://www.itl.nist.gov/fipspubs/fip46-2.htm
6 pages link to DES:
