Selenium testing introduction for beginners

Selenium is an open source technology for automating browser-based applications across different browsers and platforms.
Selenium mainly interacts with the html elements on the browser and performs actions as programmed like look and perform actions on browser manually (Clicking on button, enter text on edit box etc.).

Main components:
Selenium IDE
Selenium RC (Selenium 1)
Webdriver
Selenium GRID

Selenium IDE:
It's a Firefox add-on that will do simple record-and-playback of interactions with the browser.
it is mostly useful if we want to

• create quick bug reproduction scripts.
• create scripts to aid in automation-aided exploratory testing.

Selenium RC:
It's 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.

1. A server which automatically launches and kills browsers, and acts as a HTTP proxy for web requests from them.
2. Client libraries for your favorite computer language.

For more detailed documentation, please refer here.  

Webdriver:
It is a compact Object Oriented API when compared to Selenium1.0.
WebDriver overcomes the limitation of Selenium Rc's Single Host origin policy.
For more documentation, please refer here.

For more information on the Single host origin policy and selenium history, please refer here.

Selenium RC and WebDriver are merged into a single framework - Selenium 2.