Arduino Nano is basically a smaller version of Arduino board. There are 2 type of Arduino Nano at the time of writing:

• Arduino Nano 3.0 (ATmega328)
• Arduino Nano 2.3 (ATmega168)

More detail can be found here at official site The one I have here is v3. The obvious difference between an Uno and nano board I notice immediately is: nano board pins is facing towards bottom, where you normally mount it onto a small breadboard to use it. Next, all the pin sequence are different as shown in this picture. When uploading code from Arduino IDE, you might see some warning msg appearing at debug windows:

Sketch uses 3,010 bytes (9%) of program storage space. Maximum is 32,256 bytes. Global variables use 203 bytes (9%) of dynamic memory, leaving 1,845 bytes for local variables. Maximum is 2,048 bytes.

It means you actually scarify the memory space in exchange for the small footprint. So far I have used it for digital out & analog in and behave just the same with uno.