Selenium RC is a popular UI automation library for automating browsers. Selenium RC uses a generic form of JavaScript called Selenium Core to perform automation. However, this should comply with a security policy called the same-origin policy. The same-origin policy is a security measure that prevents website scripts from accessing the scripts of other websites. For example, JavaScript present on Google cannot access or communicate with JavaScript present on Yahoo. Three things are checked for the same-origin policy: the protocol, domain, and port. If these three things match, then only the request can be said as being one from the same domain.
Selenium Core was introduced by Jason Higgins; It was nothing but a JavaScript program. Prior to Selenium RC, IT people had to install both Selenium Core and the entire web application on their local machine to make the virtual appearance as though the requests were coming from the same domain. Selenium RC introduced the RC server, which acted as a HTTP proxy and handled the requests between the web application and Selenium Core.