Arduino is a tool for making computers that can sense and control more of the physical world than your desktop computer.
Arduino is a small circuit board with an Atmel MicroController chip and other parts.  Arduino is intended for use by both non-technical people with no previous programming experience and seasoned pros who love to tinker. Arduino was developed in Italy by Massimo Banzi and a group of people who believed Hardware and Software should be "Open Source" and available to everyone.