A cipher (pronounced SAI-fuhr) is any method of encrypting text (concealing its readability and meaning). It is also sometimes used to refer to the encrypted text message itself although here the term ciphertext is preferred. Its origin is the Arabic sifr, meaning empty or zero. In addition to the cryptographic meaning, cipher also means (1) someone insignificant, and (2) a combination of symbolic letters as in an entwined weaving of letters for a monogram.
Some ciphers work by simply realigning the alphabet (for example, A is represented by F, B is represented by G, and so forth) or otherwise manipulating the text in some consistent pattern. However, almost all serious ciphers use both a key (a variable that is combined in some way with the unencrypted text) and an algorithm (a formula for combining the key with the text). A block cipher is one that breaks a message up into chunks and combines a key with each chunk (for example, 64-bits of text). A stream cipher is one that applies a key to each bit, one at a time. Most modern ciphers are block ciphers.