This document provides an overview of the Selenium automation tool, including its components and how they work. Selenium is an open source browser automation tool that supports Java, C#, Ruby, Perl, Python and PHP. It has several components, including Selenium IDE for recording and replaying tests within Firefox, Selenium RC which provides a server to control browsers, and Selenium WebDriver which addresses some limitations of RC with a simpler object-oriented API. The document then discusses each component in more detail and provides an example of a first WebDriver script in Java.