What is an Arduino ? 
Arduino is a tool for making devices that can sense and control the physical world. Arduino can be used to develop interactive objects, taking inputs from a variety of switches or sensors, and controlling a variety of lights, motors, and other physical outputs. Arduino projects can be stand-alone, or they can be communicate with software running on your computer. The boards can be assembled by hand or purchased pre-assembled as do-it-yourself kits; the software for uploading programs into the board (open-source IDE) can be downloaded for free.
Why Arduino ??
There are many other microcontrollers and microcontroller platforms available for
physical computing. Parallax Basic Stamp, Netmedia's BX-24, Phidgets, MIT's
Handyboard, and many others offer similar functionality. All of these tools take the
messy details of microcontroller programming and wrap it up in an easy-to-use
package. Arduino also simplifies the process of working with microcontrollers, but it
offers some advantage for teachers, students, and interested amateurs over other
systems:
Inexpensive
Cross-platform
Simple, clear programming environment
Open source and extensible software
Open source and extensible hardware
Watch this video tutorial from MIT to have a better idea :
Further tutorials available here : http://arduino.cc/en/Tutorial/HomePage
