Overview:

The Arduino Nano is a small, complete, and breadboard-friendly board based on the ATmega328.  It has more or less the same functionality of the Arduino Duemilanove, but in a different package. It lacks only a DC power jack, and works with a Mini-B USB cable instead of a standard one.  See schematic below.

Specifications:

• Microcontroller:  ATmega328
• Operating Voltage (logic level):  5 V
• Input Voltage (recommended):  7-12 V
• Input Voltage (limits):  6-20 V
• Digital I/O Pins: 14 (of which 6 provide PWM output)
• Analog Input Pins:  8
• DC Current per I/O Pin:  40 mA
• Flash Memory:  32 KB of which 2 KB used by bootloader
• SRAM:  2 KB (ATmega328)
• EEPROM:  1 KB (ATmega328)
• Clock Speed:  16 MHz
• Dimensions:  0.73" x 1.70"

Power:

The Arduino Nano can be powered via the Mini-B USB connection, 6-20V unregulated external power supply (pin 30), or 5V regulated external power supply (pin 27). The power source is automatically selected to the highest voltage source.

The FTDI FT232RL chip on the Nano is only powered if the board is being powered over USB. As a result, when running on external (non-USB) power, the 3.3V output (which is supplied by the FTDI chip) is not available and the RX and TX LEDs will flicker if digital pins 0 or 1 are high.

More information on Arduino Nano here.