What is the Arduino Nano? It’s basically a surface mount breadboard embedded version with integrated USB. The size of it is small, yet it’s complete and most importantly user friendly! In case you were wondering, it’s has everything that Diecimila has electrically, yet having more analog inputs and on board +5V AREF jumper!
On the other hand, it’s physically missing the power jack as well as the power select jumper.
But why doesn’t it need a power select jumper? It’s because the Nano automatically senses and switches to a higher potential source of power.