Selenium has many projects that combine to make a versatile testing system.
Selenium IDE is a Firefox add-on that makes it easy to record and playback tests in Firefox 2+. You can even use it to generate code to run the tests with Selenium Remote Control.
Selenium Remote Control is a client/server system that allows you to control web browsers locally or on other computers, using almost any programming language and testing framework.
Selenium WebDriver can drive a browser natively either locally or on remote machines.
Selenium Grid takes Selenium Remote Control to another level by running tests on many servers at the same time, cutting down on the time it takes to test multiple browsers or operating systems.