http://www.pebbleroad.com/article/creating_maintaining_a_web_style_guide/

This article describes how to create and maintain a web style guide to better manage your organization’s web presence.

A web style guide is a key deliverable at the end of a website design/redesign process. It is a document (usually a subsite in itself) that describes what needs to be done to maintain and grow the website according to a common set of principles derived during the design or redesign process.