Here you'll find the documentation for The W3C MarkUp Validation Service. What little there is of it; additions eagerly accepted!:-)
A quick intro and help to get you started, with answers to a few Frequently Asked Questions. Read this first.
The Users Guide for the W3C Validator.
A quick introduction to SGML concepts as they apply to the Validator with links to further information.
Explanations of common error messages.
Information about the Validator for programmers.