Selenium is a suite of tools for automated web testing. It is composed of;
Selenium IDE(Integrated Development Environment); a Firefox plugin that works 
for recording and playing back.
Selenium RC(Remote Control) (1.0) ; is a test tool and is used to work on JS to
automate the web application.
WebDriver (2.0); is a web automation framework and allows you to execute your
tests in different browsers. 
Selenium Grid; allows tests to run in parallel across multiple machines.