Arduino Uno is a microcontroller board based on the ATMega328 microcontroller. It has 14 digital input/output pins of which 6 can be used as PWM outputs, 6 analog inputs, a 16 MHz crystal oscillator, a USB connection, a power jack, an ICSP header, and a reset button. To get started using your board, simply connect it to a computer with a 1.5m USB Cable Type A to B or power it with an external power supply from the power jack.
Arduino Uno differs from all preceding boards in that it does not use the FTDI USB-to-serial driver chip. Instead, it features the ATMega16U2 programmed as a USB-to-serial converter.
Arduino Uno can be powered via the USB connection or with an external power supply. The power source is selected automatically. External power can come either from an AC-to-DC adapter or battery. The adapter can be connected by plugging a 2.1mm center-positive plug into the board's power jack. Leads from a battery can be inserted in the Gnd and Vin pin headers of the POWER connector. The board can operate on an external supply of 6 to 20 volts.
Arduino Uno can be programmed with the Arduino software. ATMega328 on the Arduino Uno comes preburned with a bootloader that allows you to upload new code to it without the use of an external hardware programmer.
Arduino Web Site