user10289: Can an Arduino UNO and Arduino Leonardo do the same things?
Some of the examples in the Arduino software say to use Leonardo e.g. KeyboardMessage.

user4143: No, the Uno and the Leonardo cannot do the same thing.  The Leonardo is a single MCU directly connected to the USB port. It is down to the software on that MCU as to how it communicates (what it enumerates as) on the USB bus.
The Uno, conversely, is a dual MCU system. It has the "main" MCU (the ATMega328P) which is what you program, and that communicates to the USB port via another MCU.  The two MCUs are connected using TTL RS-232 (UART), so the only communication you have with your computer is through the Serial object.
There are benefits and problems with both approaches.  The benefits of the Leonardo are that you can emulate keyboards, mice, etc.  The down side is that the whole USB communication is more fragile and is at the mercy of the user.  The Uno's benefits are that the USB serial is always connected and available regardless of the state of the main MCU, which is much more friendly from the PC's point of view.  The down side is that it severely limits your communication options.

10289: <p>Can an Arduino UNO and Arduino Leonardo do the same things?</p>

<p>Some of the examples in the Arduino software say to use Leonardo e.g. <a href="http://arduino.cc/en/Tutorial/KeyboardMessage" rel="nofollow">KeyboardMessage</a>.</p>
 4143: <p>No, the Uno and the Leonardo cannot do the same thing.  The Leonardo is a single MCU directly connected to the USB port. It is down to the software on that MCU as to how it communicates (what it enumerates as) on the USB bus.</p>

<p>The Uno, conversely, is a dual MCU system. It has the "main" MCU (the ATMega328P) which is what you program, and that communicates to the USB port via another MCU.  The two MCUs are connected using TTL RS-232 (UART), so the only communication you have with your computer is through the Serial object.</p>

<p>There are benefits and problems with both approaches.  The benefits of the Leonardo are that you can emulate keyboards, mice, etc.  The down side is that the whole USB communication is more fragile and is at the mercy of the user.  The Uno's benefits are that the USB serial is always connected and available regardless of the state of the main MCU, which is much more friendly from the PC's point of view.  The down side is that it severely limits your communication options.</p>
