Arduino is an open-source electronic prototyping platform which is based on very easy and intuitive to use hard- and software. Originally the Arduino platform started as a student-project in Italy and after the wiring and design was finished the whole Arduino platform was made available to the open source community. Most Arduino platforms are based on a more or less powerful Atmel AVR microcontroller which can be programmed by the Arduino programming language.
If you want to get some more information about the Arduino, visit http://www.arduino.cc/ !