This is the new Arduino Uno (R3) based on Atmega328 microcontroller. This is an improved version of Arduino Uno that uses an ATmega16U2 as a USB-to-serial converter instead of FTDI USB to serial driver chip and a stronger Reset circuitry than previous version of Arduino Uno boards.
It has 14 digital input/output pins (of which 6 can be used as PWM outputs), 6 analog inputs, a 16 MHz ceramic resonator, a USB connection, a power jack, an ICSP header, and a reset button. It contains everything needed to support the microcontroller; simply connect it to a computer with a USB cable or power it with an AC-to-DC adapter or battery to get started.
The SDA and SCL pins are added next to the AREF pin and another two new pins are placed near the RESET pin. One is the IOREF that allow the shields to adapt to the voltage provided from the board. The second pin is not connected and is reserved for future purposes.
There are a large variety of shields available for the Arduino boards which allow adding custom features to monitor and control the world around it.