Selenium is a tool for automating web application testing. It supports several components including Selenium IDE for recording tests in Firefox, Selenium RC for running tests across browsers, and Selenium 2 which uses the WebDriver API to control browsers natively. When integrating Selenium with Drupal, tests can be written using the DrupalSeleniumWebTestCase class which provides methods to interact with Drupal and Selenium. Tests are run by starting a headless Xvfb server before launching the Selenium server from the command line.