Number of software's have been created as web based applications (applications that are executed in a web browser) now-a-days. Quality is the main aspect that needs to be assured before the software's are delivered to the customers. Many testing tools are available in market for testing of these software's. Selenium is one such tool available. Selenium RC tool, one of the components of selenium available is used to create automated test cases for web based applications. This paper gives an introduction of selenium and its components.