Selenium is an open-source tool primarily used for automated testing of web applications. It provides a suite of tools and libraries that enable developers to create and execute tests for web applications by simulating interactions with the browser.

The main component of Selenium is the Selenium WebDriver, an interface that allows for controlling and interacting with various browsers such as Chrome, Firefox, Safari, etc. Developers can use WebDriver to write scripts that automatically perform actions like clicking, filling out forms, navigating through pages, etc. These scripts can then be executed repeatedly to ensure that a web application functions properly and does not have any defects.

Selenium supports multiple programming languages like Java, Python, C#, Ruby, etc., allowing developers to write tests in their preferred language. It's an extremely popular tool in software development, particularly in the realm of automated testing of web applications, as it enhances the efficiency and accuracy of test runs and reduces the need for manual testing.