Wiki: GPIO

From raspberrypi.org's GPIO page:

A powerful feature of the Raspberry Pi is the row of GPIO (general-purpose input/output) pins along the top edge of the board. A 40-pin GPIO header is found on all current Raspberry Pi boards (unpopulated on Pi Zero and Pi Zero W).

The GPIO header is the main set of input/output connections on a Raspberry Pi, apart from power supply, USB, HDMI, camera, audio and network connections. It consists of two rows of twenty pins (forty pins in total) along the top of the Raspberry Pi circuit board:

GPIO pins on a Raspberry Pi A+

Pinout#

The GPIO pins have two labelling schemes, one based on the physical pin location on the header; the other, called the BCM (Broadcom Pin Number), based on their usage. GPIO software libraries force you to select which pin numbering scheme to use (e.g., BCM pin 7 is physical pin 26, BCM pin 12 is physical pin 32).

Physical pin 1 is at lower left, pin 2 upper left, pin 39 at lower right, pin 40 at upper right.