The Uno differs from all preceding boards in that it does not use the FTDI USB-to-serial driver chip. Instead, it features the Atmega16U2 (Atmega8U2 up to version R2) programmed as a USB-to-serial converter.
Revision 2 of the Uno board has a resistor pulling the 8U2 HWB line to ground, making it easier to put into DFU mode

Revision 3 of the board has the following 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.
• Atmega 16U2 replace the 8U2.

Features:

• ATmega328 microcontroller
• 5V operating voltage
• 7-12V input voltage
• 6-20V input voltage limits
• 14 digital I/O pins (of which 6 PWM output)
• 6 analog input pins
• 40mA DC current per I/O pin
• 50mA DC current for 3.3V pin
• 32KB flash memory
• 2KB SRAM
• 1KB EEPROM
• 16MHz clock speed