Selenium is an open-source browser automation framework that enables programmatic control of web browsers across multiple platforms and languages. Originally created in 2004, it has evolved into the de facto standard for web application testing and automation, with Selenium WebDriver conforming to the W3C WebDriver specification since 2018. At its core, Selenium allows you to locate elements on a page, interact with them just as a user would, and verify behavior—making it indispensable for end-to-end testing, web scraping, and repetitive browser tasks. The key mental model: Selenium doesn't "see" the page like a human does; it navigates the DOM tree (the page's underlying structure), so mastering locators and understanding how pages load asynchronously are essential to writing robust, non-flaky automation.