Arduino Leonardo Board

The Leonardo looks like an Uno and accepts the same shields, but swaps the ATmega328P for an ATmega32U4 that handles USB directly. That means it can act as a USB keyboard or mouse — plug it into a computer and it can type, click and move the cursor. It also has more analogue-capable pins than an Uno. One behavioural difference worth knowing: the serial port resets differently, so a sketch may start running before the Serial Monitor connects and you can miss the first few lines of output. Native USB lets it emulate a keyboard or mouse; Uno form factor accepts standard shields; 12 analogue-capable inputs versus the Uno's 6; Single chip handles both USB and your program