ARDUINO MEGA 2560 REV3 is a microcontroller board based on the ATmega2560. It has 54 digital input/output pins (15 of which 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. It contains everything you need to support the microcontroller; simply connect it to a computer with a USB cable or power it with an AC/DC adapter or battery to get started. The Mega 2560 board is compatible with most shields designed for the Arduino Uno and earlier Arduino Duemilanove or Arduino Diecimila boards. ARDUINO MEGA 2560 REV3 is an upgrade to the Arduino Mega.
If you are interested in boards with similar functionality, on Arduino you can find:
Arduino Uno Rev 3
Arduino Nano
Arduino DUE without headers