A brand guide is a detailed description of how to format, design and write corporate documents to ensure consistancy.
As a marketing manager it can be a challenge to ensure consistency throughout all of the corporate assets, when many marketeers, designers and other employees can create corporate documents. A brand guide describes how your brand should be presented in these documents. The use of (for instance) typography, logo variations, orthography and color palettes can be explained in detail. When used correctly, this will help adding recognition and value to the brand.