Unlike most computers, a Raspberry Pi has a series of General Purpose Inputs and Outputs (GPIOs) that allow you to interact with the world outside. These are the metal pins that stick up next to the SD card. You can use them a bit like programmable switches to turn things on and off, or you can use them to get information from other sources. In short, they allow you to expand your Pi in any way you want. They're widely used by digital artists to create interactive displays and by robot builders to bring their creations to life. With a bit of imagination, there really is no limit to what you can achieve with the Pis GPIOs and a few components.
Since this chapter is all about controlling things outside of the Pi, you will need a bit more equipment to try the examples here. It needn't be expensive though, and you can get started for just a few pounds. Even as you improve, most of the bits you'll need are cheap and easily available both online and in hobbyist stores.
Before jumping in and building circuits, the first thing you'll need is a way of connecting to the GPIO pins on the Pi. Since you can't just connect wires straight to the pins (actually you can solder directly onto them, but it's not recommended), there are a few options for accessing them, covered in the next sections.
These are probably the simplest option. They simply fit over the top of the GPIO pins and allow you to ...