Selenium is an open source automated testing tool that is used to test web applications. It has several components, including Selenium IDE which is a Firefox extension that allows users to record, edit, and playback tests in the browser. Selenium IDE provides features like record and playback, intelligent field selection, and the ability to save tests in different formats. Selenium tests are made up of actions, accessors, and assertions commands that can be used to automate interactions with a web page like clicking elements, verifying page titles, and asserting text is present. Some commonly used Selenium commands are open, click, assertTitle, and waitForElementPresent.