A block cipher takes a fixed-length block of text of bits and a key as input and produces a -bit block of ciphertext.
If our messages exceed the block-width, then the block cipher can still be used by breaking up the plaintext. NIST defines 5 modes of operation for use with any symmetric block cipher, including triple DES and AES.
A mode of operation is a technique for enhancing the effect of a cryptographic algorithm or adapting the algorithm for an application.