From the Arduino Web Site:

What is Arduino?

Arduino is an open-source physical computing platform based on a simple i/o board, and a development environment for writing Arduino software. The Arduino programming language is an implementation of Wiring, itself built on Processing.

Original Site: http://www.arduino.cc/

Setup and Intro | Programming

Setup and Intro :

• Getting Strated
• Arduino Hardware

Programming:

• Download and Install the IDE
• Arduino Reference
• Examples