A block cipher is a method of encrypting text (to produce ciphertext) in which a cryptographic key and algorithm are applied to a block of data (for example, 64 contiguous bits) at once as a group rather than to one bit at a time. The main alternative method, used much less frequently, is called the stream cipher.
So that identical blocks of text do not get encrypted the same way in a message (which might make it easier to decipher the ciphertext), it is common to apply the ciphertext from the previous encrypted block to the next block in a sequence. So that identical messages encrypted on the same day do not produce identical ciphertext, an initialization vector derived from a random number generator is combined with the text in the first block and the key. This ensures that all subsequent blocks result in ciphertext that doesn't match that of the first encrypting.