Selenium is an open-source testing framework used for automating web applications. It provides a suite of tools for web browser automation, allowing testers to interact with web elements, simulate user actions, and verify the behavior of web applications. Selenium supports various programming languages, including Java, Python, and C#, making it versatile for different development environments. With its robust features, Selenium is widely adopted for functional and regression testing of web applications, enabling efficient and reliable test automation. Its flexibility and compatibility with multiple browsers contribute to its popularity among software testing professionals.