The other is not connected and is reserved for future purposes. One is the IOREF that allows the shields to adapt to the voltage provided from the board. In addition, there are two new pins placed near the RESET pin. The Uno R3 also adds SDA and SCL pins next to the AREF. Never fear for accidental electrical discharge, either since since the Uno R3 also includes a plastic base plate to protect it! No drivers needed for Linux or Mac (inf file for Windows is needed and included in the Arduino IDE), and the ability to have the Uno show up as a keyboard, mouse, joystick, etc. This allows for faster transfer rates and more memory. In addition to all the features of the previous board, the Uno now uses an ATmega16U2 instead of the 8U2 found on the Uno (or the FTDI found on previous generations).
