The Pro Micro is a microcontroller board based on the ATmega32U4. It has 20 digital input/output pins (of which 7 can be used as PWM outputs and 12 as analog inputs), a 16 MHz crystal oscillator and a USB mini-b connection. It has a form factor that enables it to be easily placed on a breadboard.
The Micro board is similar to the Arduino Leonardo in that the ATmega32U4 has built-in USB communication, eliminating the need for a secondary usb-serial converter. This allows the Micro to appear to a connected computer as a mouse, keyboard and game controller, in addition to a virtual COM port.