Google Doctype is an encyclopedia that can be edited by anyone who has a Google account and wants to keep it up-to-date or add new articles. The encyclopedia contains articles about web security, DOM manipulation, CSS, HTML best practices, references for HTML, DOM, CSS, complete with browser compatibility information. There's also previously-unreleased code used internally by Google that is now documented and available for anyone to use.
You could call it Wikipedia for web developers or cross-browser MSDN, but Google Doctype is a clear sign that Google wants to foster a community of developers and encourage building web application using open standards.
Mark Pilgrim, the author of "Dive into Python" and now a Googler, explains more about Google Doctype in the following interview: