DES is a block cipher. It chops any plaintext to blocks of 64 bits each, and uses the same key to encrypt each such block into a 64 bits ciphertext. The reader uses the same key to decrypt the ciphertext to its corresponding plaintext.
DES was designed by IBM in the early 70s, by work order issued by the US government. IBM never released its design notes, and some suspect that DES offers a secret backdoor to crack its secret. Still it quickly became popular around the world, and is now the most used cipher ever. DES underlies all national and international financial communication.
DES key is 56 bits long, which allows modern computers to crack it using "brute force" -- testing all possible keys until stumbling on the right one. In response banks and other users employ Triple-DES, which is constructed from three individual DES ciphers.