Components of Selenium

Selenium IDE: It is the plugin to Firefox to record and play tests in Firefox and also exports tests in different language.

Selenium-IDE has a recording feature, which will keep account of user actions as they are performed and store them as a reusable script to play back.

It also has context menu integrated with the Firefox browser, which allows the user to pick from the list of assertions and verifications for the selected location.

Exported test can be run in any browser and any platform using "selenium remote control".
Selenium RC: Selenium-RC provides an API (Application Programming Interface) and library for each of its supported languages: HTML, Java, C#, Perl, PHP, Python and Ruby.

Allows playing of scripts which are exported to different platform or OS.

This ability to use Selenium RC with a high level languague to develop test cases also allows the automated testing to be integrated with the project's automated build environment.
Selenium Grid: Selenium-Grid allows the Selenium-RC solution to scale for test suites or test suites to be run in multiple environments.

With selenium-Grid multiple instances of Selenium RC are running on various operating system and browser configurations, each of these when launching register with a hub. When tests are sent to the hub they are then redirected to an available Selenium-RC, which will launch the browser and run the test.

This allows running the tests in parallel, with the entire test suite theorietically taking only as long as to run as the longest individual test.