The LilyPad Arduino USB is a microcontroller board based on the ATmega32u4 (datasheet).
It has 9 digital input/output pins (of which 4 can be used as PWM outputs and 4 as analog inputs), an 8 MHz resonator, a micro USB connection, a JST connector for a 3.7V LiPo battery, 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 battery to get started.
The LilyPad Arduino USB differs from previous LilyPad boards in that the ATmega32u4 has built-in USB communication, eliminating the need for a separate USB-to-serial adapter.
This allows the LilyPad Arduino USB to appear to a connected computer as a mouse and keyboard, in addition to a virtual (CDC) serial / COM port.
It also has other implications for the behavior of the board; these are detailed on the getting started page.