|Arduino is a single-board open source microcontroller designed around an 8-bit Atmel AVR microcontroller, or a 32-bit Atmel ARM.
Arduino Uno rev3.0
The Arduino board exposes most of the microcontroller's I/O pins, allowing the CPU board to be connected to a variety of interchangeable add-on modules known as shields. Some shields communicate with the Arduino board directly over various pins, but many shields are individually addressable via an I2C serial bus, allowing many shields to be stacked and used in parallel.
Ethernet shield stacked on Arduino Uno
The current Arduino Uno Revision 3 provide 14 digital I/O pins, six of which can produce pulse-width modulated signals, and six analog inputs. These pins are on the top of the board, via female 0.1-inch (2.5 mm) headers.
The Arduino integrated development environment (IDE) is a cross-platform application written in Java and consists of a standard programming language compiler and a boot loader that executes on the microcontroller. A program or code written for Arduino is called a sketch and are written in C or C++.
Arduino integrated development environment
More information about Arduino can be found at: http://www.arduino.cc/