Similar to an Arduino UNO, 

can be recognized by computer as a mouse or keyboard.

The Arduino Leonardo is a microcontroller board based on the ATmega32u4 (datasheet). 

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 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 Leonardo 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;