Selenium Remote Control (RC) is a test tool that allows you to write automated web application UI tests in any programming language against any HTTP website using any mainstream JavaScript-enabled browser.
Selenium RC comes in two parts.
The server piece bundles Selenium Core, but you do not need to worry about that.
Here is a simplified architectural representation....

The Selenium Server is great for testing complex AJAX-based web user interfaces under a Continuous Integration system. It is also an ideal solution for users of Selenium Core or Selenium IDE who want to write tests in a more expressive programming language than the Selenese HTML table format customarily used with Selenium Core.