"Selenium is a test tool for web applications. Selenium tests run directly in a browser, just like real users do. It runs in Internet Explorer, Mozilla and Firefox on Windows, Linux, and Macintosh, Safari on the Mac.
The quickest way to learn Selenium is via a Firefox plugin called Selenium IDE. It is quite compelling for developing tests in and quickly trying out Selenium before choosing Selenium for your project.
There are two modes of operation for Selenium - Core and Remote Control (RC). Remote Control mode also has a related capability called Selenium Grid that allows you to throw hardware at tests to make it all faster."