Raspberry Pi - header pins and BCM pins

This image is taken from Meltwaters Raspberry PI Hardware.

import Rpi.GPIO as GPIO

#Set the GPIO to use the BCM device numbering
GPIO.setmode(GPIO.BCM)

#Set the GPIO to use the Rasperry Pi board header numbering.
GPIO.setmode(GPIO.BOARD)

BCM spec

The spec for the BCM2835 can be found here.