The Mega R3 is backwards compatible with the arduino mega - same driver, same uploading, same look and work with the existing shields but can adapt to new shields.
The Arduino Mega 2560 is a microcontroller board based on the ATmega2560. It has 54 digital input/output pins (of which 14 can be used as PWM outputs), 16 analog inputs, 4 UARTs (hardware serial ports), a 16 MHz crystal oscillator, a USB connection, a power jack, an ICSP header, and a reset button.
New Features for Arduino Mega R3
- The Mega 2560 R3 adds 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.