The Arduino Uno is the latest revision of the popular Arduino open-source electronics prototyping platform, based on the ATmega328. 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. It contains everything needed to support the microcontroller; simply connect it to a computer with a USB cable or power it with a AC-to-DC adapter or battery to get started.

The Uno differs from all preceding boards in that it does not use the FTDI USB-to-serial driver chip. Instead, it features the Atmega8U2 programmed as a USB-to-serial converter. 

That's right, folks, the Arduino Uno features not one, but two Atmel microcontrollers. 

It also runs a new bootloader, Optiboot, which takes up 1.5kb less space than STK500 (the previous bootloader) and runs at twice the speed (for faster sketch uploading).

Various lengths of USB A-B cables can be found here:

Specifications

• Atmega328
• 14 I/O pins
• 6 PWM
• 6 Analog
• 16MHz
• 32kB Flash Memory
• 2kB RAM
• Now with stickers!
  

Resources

• Arduino Website
• ATmega328 Datasheet