The ADK is based on the Mega 2560.
Similar to the Mega 2560 and Uno, it features an Atmega8U2 programmed as a USB-to-serial converter.
For information on using the board with the Android OS, see Google's ADK documentation.
New for R3:
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 both with the board that use the AVR, which operate with 5V and with the Arduino Due that operate with 3.3V. The second one is a not connected pin, that is reserved for future purposes.
Stronger RESET circuit.