Selenium is a web testing framework that can run tests in any browser. It allows test cases to be written in languages like Java, Python, and Ruby. Selenium tests can be run directly in browsers or through Selenium Remote Control (RC), which allows tests to be run on different machines. RC is useful for data-driven testing, database testing, and checking API values. Common Selenium commands include typing into fields, clicking buttons and links, waiting for pages to load, and verifying text is present.