Arduino Uno is a microcontroller board based on Atmega328. There are 16 digital I/O pins (6 of them are PWM out), 6 analog input pins, a 16 MHz crystal ascilator, an USB socket, a power socket, an ICSP connector and reset button on the board. There are the all essential parts on the board for microcontroller operations. The board can easily be connected to the computer via usb or can be used with an external adapter of battery. Arduino Uno has an Atmega16u2 (It was Atmega8u2 on the Rev 2 model) usb - serial chip for programming and communication with computer via usb. Arduino.cc made various innovations on the Rev 3 in comparison with Rev 2 to be useful and stable. The Arduino Uno board has been a referance model as both software and hardware to all the other Arduino models. You may see the historical development of the Arduino boards by hardware index.