This tutorial will walk you through the basic setup of your own WebDriver server/client system, it will provide you with the basic knowledge on how WebDriver works, but aims not be a full blown browser testing solution etc.
WebDriver is a W3C editors draft for writing automated tests of websites. It aims to mimic the behaviour of a real user, and as such interacts with the HTML of the application.
All implementations of WebDriver that communicate with the browser use the JSON WireProtocol. This wire protocol defines a RESTful web service using JSON over HTTP.