"A bible for writers is a reference document used for information on a story's characters, settings and other elements. They are commonly used in mediums with multiple writers, such as comic books and television series, where new writers and freelancers are often referred to it when writing scripts for the show to ensure continuity with previous episodes, but are also used by individual writers for books and movies, to keep track of details.
Show bibles are an optional part of the television series development process: in some cases they are sales documents, created to help people at a network or studio understand a new show, in some cases they are given to new writers to help them gain that same understanding, and in rare cases, they are meticulously maintained archives of everything known about a show's characters."
The above description is taken from this Wikipedia page – http://en.wikipedia.org/wiki/Bible_(writing) – which also has links to bibles for shows including Freaks And Geeks, He-Man and Transformers.
And now the kicker – the show bible for The Wire is on the interwebs, here: