Arduino Uno Rev3 is a circuit board based on the Atmega328 microcontroller. It has 14 digital inputs/outputs (of which 6 can be used as PWM outputs), 6 analog inputs, a 16 MHz crystal oscillator, a USB connection, a power jack, an ICSP header, and a reset button.
It contains everything needed to support the microcontroller; to get started, simply connect it to a computer via a USB cable or power it with an AC/DC adapter or a battery. The UNO differs from all previous boards in that it does not use the FTDI USB-to-serial converter but an Atmega16U2 microcontroller programmed as a USB-to-serial converter.

Difference between Arduino Uno and Arduino Uno REV3

Added SDA and SCL pins near the AREF pin and two new pins near the reset pin, including IOREF which allows the shields to adapt their voltage level to that of the board. Replaced the Atmega8U2 microcontroller with Atmega16U2.

The Arduino Uno REV3 package

Technical features

• Microcontroller: ATmega328
• Operating voltage: 5V
• Diet:
  – from 7 to 12 V (via plug)
  – 5V (via USB port)
• Supply voltage (limits): 6-20V
• Digital Inputs/Outputs: 14 (of which 6 can be used as PWM outputs)
• Analog Inputs: 6
• DC current per I/O pin: 40 mA
• DC current for 3.3V pin: 50mA
• Flash Memory: 32 kB (of which 0.5 kB used by the bootloader)
• SRAM: 2kB
• EEPROM: 1KB
• Clock Speed: 16 MHz