Selenium is an open source (free) automated testing suite to test web applications. It supports different platforms and browsers. It has gained a lot of popularity in terms of web-based automated testing and giving a great competition to the famous commercial tool HP QTP (Quick Test Professional) AKA HP UFT (Unified Functional Testing).
Selenium is a set of different software tools. Each tool has a different approach in supporting web based automation testing.
It has four components namely,
Selenium IDE (Selenium Integrated Development Environment)
Selenium RC (Selenium Remote Control)