Selenium WebDriver is an open-source tool that is used to automate web browsers. It is popular among developers who need to test web applications across multiple browsers. Selenium WebDriver provides a powerful and reliable automation framework that makes it easy to perform browser-based regression testing. It is designed to be highly flexible and allows developers to write tests in a variety of programming languages, including Java, Python, and C#. With Selenium WebDriver, developers can write tests that simulate end user actions on the web application, such as clicking on links, filling out forms, and verifying the results. This makes it easy to quickly catch and fix errors before the application is released. Selenium WebDriver is becoming increasingly popular among developers, as it provides a powerful and reliable way to automate web browser tests.