Just like the Duemilanove only smaller. The Arduino Nano microcontroller is very similar to the Duemilanove but comes in a considerably smaller package and is suitable for mounting on a breadboard. It has the same Atmega328 chip, connects with a USB cable and has all the same input/output.
Designed for use on breadboards the Arduino Nano is mostly the same as the Arduino Duemilanove but smaller.
The Nano easily connects to your computer via a Mini-B USB cable to transfer programs 'sketches' for running on the board itself or for sharing data between the board and pc. It features 14 digital IO pins (6 can be used as PWM outputs), 6 analog inputs, USB connection, 16 MHz crystal oscillator, a power jack, reset button and ICSP header.
Power can be supplied directly from the USB cable or via pin 30 (6-20V) from a battery or DC power supply with the Nano automatically selecting the power source.
The main differences between the Nano and the Duemilanove are:
Software for programming the Nano is available free from the Arduino web site and runs on Windows, Mac or Linux.
Part of the secret to the Arduino success is that it makes microcontrollers accessible to people without previous experience. The USB support and bootloader on the Nano make it easy to transfer programs to the board without any additional hardware unlike many other boards, plus the large community of enthusiests sharing program examples and hardware setup ideas makes gettings started easy.
For more details on the Nano please check this Arduino web page.