1. Selenium Integrated Development Environment (IDE)
Selenium IDE is implemented as Firefox extension which provides record and playback functionality on test scripts.
2. Selenium Remote Control
Selenium RC allows testers to write automated web application UI test in any of the supported programming languages. It also involves an HTTP proxy server which enables the browser to believe that the web application being tested comes from the domain provided by proxy server.
RC is outdated now.
3. Selenium WebDriver
Selenium WebDriver is the successor to Selenium RC and is by far the most important component of Selenium Suite. SeleniumWebDriverprovides a programming interface to create and execute test cases. Test scripts are written in order to identify web elements on web pages and then desired actions are performed on those elements.
Selenium WebDriver performs much faster as compared to Selenium RC because it makes direct calls to the web browsers. RC on the other hand needs an RC server to interact with the web browser.