this tutorial will guide you to understand which component is best suitable for our projects need.

Selenium IDE - when you don't have any programming knowledge and you want simple test cases and test suits to be imported in Selenium RC or grid, to know more about locators and functions such as type, open, xpath, css locators when you want to test the application against Firefox only then you can select the Selenium IDE

Selenium RC - when you want to apply more conditions on your test cases, and want to test your app against different browsers also want to test the app with complex AJAX-based scenarios. you can use Selenium RC. It is also helpful when you want to test your application against a new browser that supports JavaScript.

WebDriver - when you want to work on a certain programming language, when you want to execute tests on the HtmlUnit browser and when you want customize test results you can use WebDriver.
Selenium Grid when you want lesser time in testing and when you want to to test the app across multiple browsers and operating systems, also when you have large test suite to execute then you will choose Selenium Grid