This is the Adafruit METRO with headers. It comes with headers soldered on. It’s a fully assembled and tested board with through-hole headers attached. If you don’t want a Metro with the headers attached for super-slimness, check out our Metro without Headers.
METRO is the culmination of years of playing with AVRs: we wanted to make a development board that is easy to use and is hacker friendly. At the heart is an ATmega328P, with 32KB of flash and 2KB of RAM, running at 16 MHz. It comes with the Optiboot bootloader already installed, and is Arduino IDE compatible.
Key Features

ATmega328 microcontroller with Optiboot (UNO) Bootloader
USB Programming and debugging via the well-supported genuine CP2104
Input voltage: 7-9V (a 9VDC power supply is recommended)
5V regulator can supply peak ~800mA as long as the die temp of the regulator does not exceed 150*C
3.3V regulator can supply peak ~150mA as long as the die temp of the regulator does not exceed 150*C
5V logic with 3.3V compatible inputs, can be converted to 3.3V logic operation
20 Digital I/O Pins: 6 are also PWM outputs and 6 are also Analog Inputs
6-pin ICSP Header for reprogramming
32KB Flash Memory – 0.5K for bootloader, 31.5KB available after bootloading
16MHz Clock Speed
Compatible with “Classic” and “R3″ Shields
Adafruit Black PCB with gold plate on pads
53mm x 71mm / 2.1″ x 2.8″
Height (w/ barrel jack): 13mm / 0.5”
Weight: 19g