WebDriver is now a W3C Recommendation
The WebDriver spec defines a set of interfaces and a wire protocol that are platform and language-neutral and that allow out-of-process programs to remotely control a browser in a way that emulates the actions of a real person using the browser. It is primarily intended to allow web authors to write tests that automate a user agent from a separate controlling process, but may also be used in such a way as to allow in-browser scripts to control a — possibly separate — browser.
There are already implementations of the WebDriver standard available for every major desktop browser, and language bindings are offered by a number of projects, notably Selenium. Having a standard way to automate interaction with a browser — a way that works across different browsers and browser engines — is a big win for Web developers in helping ensure their Web applications work in the best way they should for their users.