A block cipher is a type of symmetric-key encryption algorithm that transforms a fixed-length block of plaintext (unencrypted text) data into a block of ciphertext (encrypted text) data of the same length. This transformation takes place under the action of a user-provided secret key. Decryption is performed by applying the reverse transformation to the ciphertext block using the same secret key. The fixed length is called the block size, and for many block ciphers, the block size is 64 bits. In the coming years the block size will increase to 128 bits as processors become more sophisticated.
For those with a mathematical background (see Appendix A): Since different plaintext blocks are mapped to different ciphertext blocks (to allow unique decryption), a block cipher effectively provides a permutation (one to one reversible correspondence) of the set of all possible messages. The permutation effected during any particular encryption is of course secret, since it is a function of the secret key.
When we use a block cipher to encrypt a message of arbitrary length, we use techniques known as modes of operation for the block cipher. To be useful, a mode must be at least as secure and as efficient as the underlying cipher. Modes may have properties in addition to those inherent in the basic cipher. The standard DES modes (see Question 3.2.3) have been published in FIPS 81 [NIS80] and as ANSI X3.106 [ANS83]. A more general version of the standard [ISO92b] generalized the four modes of DES to be applicable to a block cipher of any block size. The standard modes are Electronic Code Book (Question 22.214.171.124), Cipher Block Chaining (Question 126.96.36.199), Cipher Feedback (Question 188.8.131.52), and Output Feedback (Question 184.108.40.206).
More information about block ciphers and the various available algorithms can be found in almost any book on contemporary cryptography.