The pro Micro is based on the ATMega32U4 Chip and therefore has native USB so it will work with all computers, USB serial debugging, and a more reliable bootloader.
Has four channels of 10-bit ADC, 5 PWM pins 12 Digital in/out pins as well as hardware serial connections Rx and Tx, and the board runs at 16MHz 5VDC.
Give you all the functionalities of a popular dev board in a much smaller footprint and great for projects with USB.
Quick Specs:
ATmega32u4 onboad chip in QFN package
5V power and logic, 16MHz clock rate, 2KB RAM and 28K FLASH
USB bootloader with a nice LED indicator, AVR109 compatible (same as Leonardo, etc)