The Arduino Micro is a small microcontroller board based on the ATmega32u4. It has 20 digital input/output pins (of which 7 can be used as PWM output and 12 as analog input), a 16Mhz crystal, a micro USB connection, an ICSP header and a reset button. The board contains everything to let you get started right away.
The ATmega32UA has a built-in USB which makes the extra processor for the USB connection unnecessary. It is also possible to connect the Arduino Micro as a mouse or keyboard to a PC.