why is the Leonardo different? Because it uses the ATmega32u4. This processor has built in USB communication which eliminates the need for a secondary USB to serial converter. The ATmega32u4 creates a virtual (CDC) COM port on your computer every time it runs its bootloader. Since it’s virtual, it can also behave like an HID (Human Interface Device) meaning the Leonardo can “act” like a keyboard or mouse, opening it up to a whole new range of projects. This processor also has additional I/O capabilities, allowing pins 4, 6, 8, 9, 10, and 12 to be used as analog inputs (12 total vs. the UNO’s 6). In addition, the Leonardo has one additional PWM pin (13) and all 20 I/O pins can be used as digital pins.