The Raspberry Pi is an incredible little computer made up of a components and connectors on an unfeasibly small printed circuit board that is approximately the dimensions of a credit card. Depending upon your intended use, your Rasberry Pi may have a keyboard, monitor and mouse added. Regardless of your intended use, you are going to need an SD (Secure Digital) card on which to install an operating system.
This tutorial describes how to install an operating system onto an SD card for use in your Raspberry Pi.