What is this product about? Arduino in general is an open-source physical computing platform based on a simple i/o board and a development environment that implements the Processing/Wiring language. Arduino can be used to develop stand-alone interactive objects or can be connected to software on your computer (e.g. Flash, Processing, MaxMSP). The open-source IDE can be downloaded for free (currently for Mac OS X, Windows, and Linux).
In case you were wondering, the Leonardo is Arduino's first development board to use one microcontroller with built-in USB, and the thing that allows it to be cheaper and simpler is basically by using the ATmega32U4 as its sole microcontroller, Also, because the 32U4 is handling the USB directly, code libraries are available which allow the board to emulate a computer keyboard, mouse, and more using the USB-HID protocol, which is super cool!
It has 20 digital input/output pins (of which 7 can be used as PWM outputs and 12 as analog inputs), a 16 MHz crystal oscillator, a micro 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, and if you're not sure which Arduino or Arduino-compatible board is right for you?
[Product description and pictures courtesy of SparkFun.com]