Selenium is an open source tool used for automating web application testing. It was created in 2004 by Jason Huggins and supports recording and playback of test cases in browsers like Firefox. Selenium has four main components - Selenium IDE for recording and playback of tests, Selenium Remote Control for running tests on multiple browsers, Selenium WebDriver for direct browser control, and Selenium Grid for parallel testing on different machines. The latest version of Selenium is 3.0.