The Arduino Leonardo is a simple yet very versatile Arduino board based on the atmega32u4 processor.
It has a few interesting features:
Single processor architecture. There is only one microcontroller that communicates directly via usb with a computer. This makes the board cheaper and simpler
The Arduino core libraries have been enhanced to provide a USB device stack. This allows the Arduino Leonardo to emulate a computer keyboard or a mouse. In the near future we expect to release more devices like MIDI.
More analog inputs, the Leonardo has 12 of them!
More connectivity: Since the communication with the computer is done through a simulated serial port over USB, the hardware serial port on the processor is free for your applications.