The arduino UNO R3 is backwards compatible with the UNO - same driver, same uploading, same look.
- There are a few changes in the UNO, here is what they are
- ATmega16U2 instead of the 8U2 found. This allows for faster USB transfer rates.
- No drivers needed for Linux or Mac (.inf file for Windows is needed and included in the Arduino IDE)
- The ability to have the Uno show up as a keyboard, mouse, joystick, etc.
- New SDA and SCL pins next to the AREF. this is a duplication of the Analog 4 and 5 pins.
- Two new pins placed near the RESET pin. One is the IOREF that allow the shields to adapt to the voltage provided from the board. The other is a not connected and is reserved for future purposes.
- Reset button moved from center of board to corner near USB connector, allowing resets when shields are connected.
- Larger heat sink copper area for the LM117 regulator