Arduino is an open-source physical computing platform based on a simple i/o board and a development environment that implements the Processing
language. Arduino can be used to develop stand-alone interactive objects or can be connected to software on your computer (e.g. Flash, Processing, MaxMSP). The open-source IDE can be downloaded
for free (currently for Mac OS X, Windows, and Linux).
The Arduino Mega 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. It contains everything needed to support the microcontroller; simply connect it to a computer with a USB cable or power it with a AC-to-DC adapter or battery to get started. The Mega is compatible with most shields designed for the Arduino Duemilanove or Diecimila.
This is the new Arduino Mega 2560. In addition to all the features of the previous board, the Mega 2560 now uses an ATmega8U2 instead of the FTDI chip. This allows for faster transfer rates, no drivers needed for Linux or Mac (inf file for Windows is needed), and the ability to have the board show up as a keyboard, mouse, joystick, etc. It also has twice as much flash memory.
Can't decide which Arduino is right for you? Arduino buying guide