The Arduino UNO R3 is an electronic circuit based on Atmel's ATmega microcontroller, and all its designs, as well as the software needed for its operation, are distributed freely and at no cost so that it can be constructed by anyone (hence the unusual -for hardware- designation "open source").

Once constructed, it can behave like a tiny computer, as the user can connect multiple input/output units to it and program the microcontroller to receive data from the input units, process it, and send appropriate commands to the output units.