Selenium is a browser automation tool which lets you automated operations like type, click, and selection in a web page.
Selenium is a library which is available in more languages, java, C#, python, ruby, php, etc while most commercial tools are limited in their capabilities of being able to use just one language.
Selenium has four set of tools Selenium IDE, Selenium Remote Control, Selenium 2.0 or WebDriver and Selenium Grid.
Selenium is a suite of tools to automate web browsers across many platforms.
Selenium runs in many browsers and operating systems and can be controlled by many programming languages and testing frameworks.