The Micro is a microcontroller board based on the ATmega32U4 (datasheet), co-developed with Adafruit. It has 20 digital input/output pins (7 of which can be used as PWM outputs and 12 as analog inputs), a 16 MHz crystal oscillator, a micro USB connection, an ICSP header, and a reset button. It contains everything needed to support the microcontroller; simply connect it to a computer using a micro USB cable to start using it. Its form factor allows it to be easily placed on a breadboard.
The Micro board is similar to the Arduino Leonardo because the ATmega32U4 has built-in USB communication and does not require an auxiliary processor. This allows the Micro to appear as a mouse and keyboard on the connected computer, in addition to the virtual (CDC) serial/COM port.