The Raspberry Pi is a $(removed) Linux computer the size of a credit card. I'm running a Minecraft server on one (above photo). In observance of today's holiday, MAKE is running an excerpt from Matt Richardson and Shawn Wallace's excellent book, Getting Started with Raspberry Pi.
As makers, we have a lot of choices when it comes to platforms on which to build technology-based projects. Lately, microcontroller development boards like the Arduino have been a popular choice because they've become very easy to work with. But System on a Chip platforms like the Raspberry Pi are a lot different than traditional microcontrollers in many ways. In fact, the Raspberry Pi has more in common with your computer than it does with an Arduino.
This is not to say that a Raspberry Pi is better than a traditional microcontroller; it's just different. For instance, if you want to make a basic thermostat, you're probably better off using an Arduino Uno or similar microcontroller for purposes of simplicity. But if you want to be able to remotely access the thermostat via the web to change its settings and download temperature log files, you should consider using the Raspberry Pi.