Each Pi (models A+, B+, 2B, and 3B) comes with a total of 40 pins. These can be either GPIO pins or they can have some other function, as shown in the following diagram:

The 3.3V and 5V pins act only as a power supply and nothing else. We cannot control them with the Pi. Similarly, the ground pin acts as the negative terminal of this power supply. The GPIO pins are where all the action takes place since we can control them as well as read from them programmatically. The ID EEPROM pins are meant for more advanced usage and should not be manipulated unless you know exactly what you are doing.

The numbers on the pins may seem haphazard, and they are, but this is how the computer sees them. So when we refer to pin 2, we are actually referring to the one numbered 2 in the previous diagram.