The Pro Micro - with soldered headers - is similar to the Pro Mini except with an ATmega32U4 on board. The USB transceiver inside the 32U4 allows adds USB connectivity on-board - no need for an external USB interface.

This tiny board does all of the Arduino tricks:

• 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
• Voltage regulator on board - it can accept voltage up to 12VDC. If you’re supplying unregulated power to the board, be sure to connect to the “RAW” pin on not VCC.

• ATmega32U4 running at 3.3V/16MHz
• Supported from Arduino IDE v1.0.1 onward (recognised as Leonard)
• 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
• Headers soldered