Any AVR microcontroller based board which follows the standard arduino schematic and is flashed with the arduino boot-loader can be called an arduino board. The arduino is refered to as open source hardware, since the standard schematic is open to everyone and anybody can make their own version of arduino board following the standard schematic. All arduino boards should be compatible with the arduino IDE which can be used to program the arduino boards. The arduino IDE is also open source and anybody can contribute their libraries to the arduino. There is no other tool available which helps in easy prototyping like the arduino does. The arduino board has all the required circuitary to get the built-in AVR microcontroller running. The output or inputs can be taken from the boards or given to the board using convenient connectors.