The Nano is pretty much a small UNO there are very little differences between the two apart from the size difference, the lack of an external power jack and the connection pins which all can be seen in the photos below.
The Nano also has the handy advantage that is can be plugged straight in to a breadboard for super quick prototyping, something that the UNO lacks unless you have lots of jumper wires! You will notice that the all pins other than the ICSP pins are pointing down so they can be bread boarded, they are male pins rather than female ones that you will find on the UNO.
