Selenium WebDriver is a free and open-source tool used for web automation, allowing users to execute tests against various browsers. It's a collection of APIs that provide language-agnostic functionality through client drivers in languages such as Ruby, Java, Python, C#, and JavaScript. With Selenium WebDriver, teams can increase testing speed, accelerate cross-browser tests at scale, and focus on creating higher-quality code. While it has some drawbacks, such as limitations in automating certain types of tests and compatibility issues with native or mobile applications, Selenium is a flexible and scalable choice for many teams looking to start automated testing.