Selenium Rc

Selenium-Selenium is a portable software testing framework for web applications. Selenium provides a record/playback tool for authoring tests without learning a test scripting language.

Rc-Remote control

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.
-A server which automatically launches and kills browsers, and acts as a HTTP proxy for web requests from them.
-Client libraries for your favorite computer language.
Selenium Remote Control 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 IDE who want to write tests in a more expressive programming language than the Selenese HTML table format.

Selenium RC is the old version of selenium. Its powerful but has many issues as well.
The core engine of selenium RC is is based on javascript(similar to IDE). Selenium RC can be implemented in many languages such as:
- Ruby
- Java
- Pearl
- C#
- Python
- PHP

This makes it very powerful as you can write your test scripts using any one of the languages. The language you select to write your Selenium tests is independent of the language in which your website is developed.

Selenium rc api

The Selenium Server is needed in order to run either Selenium RC style scripts or Remote Selenium Webdriver ones. The 2.x server is a drop-in replacement for the old Selenium RC server and is designed to be backwards compatible with your existing infrastructure.
Selenium RC and WebDriver are two different entity, but from Selenium 2.0 release we are seeing the gap between Selenium RC and Webdriver is getting very less, it seems like the latest version of selenium proves to show a very tight integration between both Selenium RC and WebDriver.

Reference
Wikipedia
www.ITeLearn.com