The Leonardo is the newest in the Arduino family. Using the ATmega32U4 as its sole microcontroller allows it to be cheaper and simpler. Code libraries allows the board to emulate a computer keyboard, mouse, and more.
The Leonardo even supports up to 12 analog inputs! And since the communication with the computer is done through a simulated serial port over USB, the hardware serial port on the processor is free for your applications.
The Arduino Leonardo requires the installation of Arduino IDE 1.0.1 or later. User's may need to install new drivers, see the guide below.
Please read the documentation and understand the differences between the Leonardo and the UNO. Two important differences are that the Leonardo does not have the SPI pins connected to pin 10-13 and the I2C communication pins are on pins 2/3 and not 4/5 as with the Arduino UNO. Therefore shields that use SPI and I2C (TWI) will not be directly compatible.