A style guide is essentially a set of rules written to guide the style elements of a publication. It can include, for example, writing styles (punctuation, capitalization etc.) and visible styles such as color and size.
This is more than just a style guide, it's a type of manual for web design that explains many of the basics and some of the more complex issues facing web developers today. It covers the whole gamut of topics that we need to be aware of when building a site, from page design and typography to user tracking and site statistics to backups and browser safe colors (which, maybe aren't an issue any more :).
This is worth flipping thru if you have arrived at web design and development from the side door somehow.
If you have the time to click thru this guide you are sure to come across some piece of sage advice that will improve your writing style. The guide is easy to navigate from one section to the next, and cons ice and to the point, so it make for fast learning. If you are planning on doing much writing, be sure to read this guide.
Mollio is a set of HTML and CSS templates that come with a short but useful style guide that outlines the basic layout structure for the different templates. There are 5 different layouts provided, each with most of the basic content you will ever need already styled up in the CSS.
The University of Alicante in Spain provides this style guide for web page design withing the schools website. This is a decent example of a corporate site design guide. Note that parts of it are in Spanish and this guide is a little dated.
Silver Orange labs offers up a style guide, released under the Attribution-ShareAlike License from the Creative Commons. From the guide: This document outlines basic principles for adding content to the site, defines the basic style sheet classes available to content managers and how they should be used, and defines principles, guidelines, and best practices for content.