What is a style guide?
A style guide or style manual is a set of standards for the writing and design of documents, either for general use or for a specific publication, organization or field. The implementation of a style guide provides uniformity in style and formatting of a document. See Wikipedia for more.
A List Apart have a style guide that I think is quite usable, such that I now loosely base our own style guide on it for an internal wiki that contains our procedures.