Selenium is an open source automation testing tool.
Used only for web based applications.
Supports writing of testcases in different languages like Html, C#, Java, Phython, Ruby etc.
Supports the execution of test cases on multiple web browsers such as Firefox, Chrome, IE, Opera, HtmlUnit, Android, iOS .
Mainly used for functional, regression and load testing.
Released under Apache 2.0 licence.
It can be downloaded from seleniumhq.org.
Highly flexible as it provides many options for locating UI element and comparing expected test results against actual application behaviour. It can be deployed on Windows, Linux, and Macintosh.