Arduino Uno is the new successor to the Duemilanove board. With all the great features of the previous board the Uno adds faster USB transfer and can show up as a keyboard, mouse, joystick etc in your computer. This is a great introductory or rapid prototyping board.
Arduino 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).
This is the new Arduino Uno. In addition to all the features of the previous board, the Uno now uses an ATmega8U2 instead of the FTDI chip. This allows for faster transfer rates, no drivers needed for Linux or Mac (inf file for Windows is needed), and the ability to have the Uno show up as a keyboard, mouse, joystick, etc.
The Uno easily connects to your computer via a USB cable to transfer programs 'sketches' for running on the board itself or for sharing data between the board and pc. It features 14 digital IO pins (6 can be used as PWM outputs), 6 analog inputs, USB connection, 16 MHz crystal oscillator, a power jack, reset button and ICSP header.
Power can be supplied directly from the USB cable or via the power jack from a battery or DC power supply with the Uno automatically selecting the power source.
Software for programming the Uno is available free from the Arduino web site and runs on Windows, Mac or Linux.
Part of the secret to the Arduino success is that it makes microcontrollers accessible to people without previous experience. The USB support and bootloader on the Uno make it easy to transfer programs to the board without any additional hardware unlike many other boards, plus the large community of enthusiests sharing program examples and hardware setup ideas makes gettings started easy.
If you are new to electronics and/or programming then I strongly recommend Limor's Arduino tutorial as an excellent intoduction.