While the Raspberry Pi can be used as a miniature, general-purpose computer, its utility really shines when you take advantage of the general purpose I/O (GPIO) pins. The GPIO pins can be used to read digital signals, power small components, and talk to a variety of devices ranging from temperature sensors to accelerometers to radio frequency identification (RFID) readers.
Depending upon the model of the Raspberry Pi you are using, the number of available GPIO pins and the pin layout vary. Here is a quick summary of the differences you will find on different boards:
Raspberry Pi B (rev 1) 26-pin header with 17 available ...