The Uno now uses an ATMega16U2 instead of the ATMega8U2 chip. Faster transfer rates and more memory.
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.
Add SDA and SCL pins next to the AREF. In addition, there are 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. The Uno R3 works with all existing shields but can adapt to new shields which use these additional pins.

Technical Details

Product Details

Product Dimensions

Package Dimensions
Width : 4.3 inch
Height : 1 inch
Length : 4.8 inch

Item Weight: 3.2 ounce (View shipping rates and policies)
Shipping Weight: 3.2 ounce (View shipping rates and policies)

ASIN : B00MHB27BK