Selenium is a widely-used open-source tool for automating web browsers, primarily used for testing web applications. It allows developers to write scripts in multiple programming languages, such as Python, Java, and C#, to interact with web elements, simulate user actions, and automate tasks like form submissions, clicks, and navigation. Selenium supports major browsers and platforms and can run tests across different environments, making it a powerful tool for browser automation and regression testing. It's commonly used for functional testing, web scraping, and automating repetitive web-based tasks.