Selenium is an open source web automation tool which can be used for performing functional test execution. It’s been developed by SeleniumHQ. It can be used for various testing activities such as regression testing, unit testing, sanity testing etc. across various web browsers. Basically it comprises of three components:
  1. Selenium IDE (Integrated Development Environment)
  2. Selenium Server (formerly known as Selenium RC Server)
  3. Selenium Grid
It supports various scripting language such as Java, Python, Ruby, C#, etc for the automation. It can also run on various OS such as Windows, MAC and Linux. Latest version  of selenium can be download from their website: http://seleniumhq.org/download/