Description from the Arduino Uno website:
The Arduino Uno is a microcontroller board based on the ATmega328 (datasheet). It has 14 digital input/output pins (of which 6 can be used as PWM outputs), 6 analog inputs, a 16 MHz ceramic resonator, 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 Arduino Uno R3 kit includes the assembled Arduino board, four rubber bumpers, a sheet of cool stickers, and some printed documentation.
Here's what the Arduino project has to say:
Arduino is an open-source electronics prototyping platform based on flexible, easy-to-use hardware and software. It's intended for artists, designers, hobbyists and anyone interested in creating interactive objects or environments.
This kit is open source hardware. Arduino makes the hardware source files like the schematic and the pcb files available. The schematic and pcb files are available on the Arduino Uno R3 page.