In Italian,the word "Uno" means one.It was named to remember the creation of Arduino 1.0.Now for most people who have interest in the Arduino Uno R3 projects,Arduino UNO R3 is the most advanced board in the Arduino world.
The Arduino Uno R3 board has six new features:
1.0 pinout: added SDA and SCL pins that are near to the AREF pin and two other new pins placed near to the RESET pin, the IOREF that allow the shields to adapt to the voltage provided from the board. In future, shields will be compatible with both the board that uses the AVR, which operates with 5V and with the Arduino Due that operates with 3.3V. The second one is a not connected pin, that is reserved for future purposes.
Stronger RESET circuit.
The Arduino Uno R2 has an ATMEGA8U2 USB microcontroller on board while the Arduino Uno R3 development board has an ATMEGA16U2 on it.It's more advanced than R2.
The arduino uno R3 board adds a diode across the USB ATMEGA reset pin pull-up
The USB Chip Position is different.On the Arduino Uno R2 board, the ATMEGA8U2 is positioned at 45 degrees. This is changed back to square position on the R3 board.
The Arduino Uno R2 has a LED and resistor connected in series on Arduino pin 13. The Arduino Uno R3 board protect this LED/resistor through a unity gain op-amp. This is the spare op-amp that was unused on previous boards.