The Arduino nano is a very nice board, it has the same microcontroller as an Uno, all of its I/O come out to headers, it uses mini usb and its small size make it ideal for projects where space is an issue. its also generally slightly cheaper than Uno making it a popular choice.
There are a couple of minor problems though, if you have a collection of shields for your Arduino Uno then using these with a nano can be tricky an additional issue that as the nano is USB powered then there is a lack of power available due to the amount available through the USB bus and also the limitation of the on board voltage regulator. The nano has no external power adaptor.
To solve this there are a couple of solutions, what you need to buy is a Nano-Uno shield adaptor. Here is a picture of one that I bought
As you can see there are coloured connectors for the shield, you can also see the power connector fitted so you can use an external supply. Headers are supplied which could be populated but depending on the size of these headers then they may be close to shorting on a shield if fitted, double check if you want to fit these.
You can get one of these useful boards for under $10
Nano-UNO Shield Adapter Nano Development Board for Arduino – $9.79