The Pro Micro is similar to the Pro Mini except with an ATmega32U4 on board.

The USB transceiver inside the 32U4 allows us to add USB connectivity on-board and do away with bulky external USB interface.

This tiny little board does all of the neat-o Arduino tricks that you're familiar with: 4 channels of 10-bit ADC, 5 PWM pins, 12 DIOs as well as hardware serial connections Rx and Tx. Running at 16MHz and 5V, this board will remind you a lot of your other favorite Arduino-compatible boards but this little guy can go just about anywhere. 

NB. There is a voltage regulator on board so it can accept a voltage of upto an ABSOLUTE MAXIMUM of 12.00VDC,  but only on the RAW pin and NOT on the VCC. The VCC is a 5V output. DO NOT SUPPLY 12V TO THIS PIN. (To be safe rather use a maximum of 9VDC to the RAW pin)

• ATMega 32U4 running at 5V/16MHz
• Supported under Arduino IDE v1.0.1 and up
• On-Board micro-USB connector for programming
• 4 x 10-bit ADC pins
• 12 x Digital I/Os (5 are PWM capable)
• Rx and Tx Hardware Serial Connections
• Dimensions : 1.3 X 0.7mm