Selenium is an open source automated testing tool used for web applications. It was originally developed in 2004 as a JavaScript library but has since evolved. The current versions are Selenium WebDriver, which allows direct communication with browsers, and Selenium Grid for distributed testing across multiple browsers and machines. Selenium can automate tests written in various programming languages like Java, Python, and C#. It supports cross-browser testing and provides tools like Selenium IDE for rapid test prototyping without coding.