The Leonardo R3 Board Micro-USB compatible with Arduino is a micro-controller board base on the ATmega32u4. It has 20 digital input/output pins (of which 7 can be used as PWM outputs and 12 as analog inputs); a 16 MHz crystal oscillator, a micro USB connection, a power jack, an ICSP header, and a reset button.
It contains everything need to support the microcontroller; simply connect it to a computer with a USB cable or power it with an AC-to-DC adapter or battery to get started.
The Leonardo R3 Board differs from all preceding boards in that the ATmega32u4 has built-in USB communication; eliminating the need for a secondary processor.
This allows the Leonardo to appear to a connected computer as a mouse and keyboard; in addition to a virtual (CDC) serial / COM port. It also has other implications for the behavior of the board.