About Raspberry Pi
Raspberry Pi is a small board that has a computer on it. The computer on the Raspberry Pi is the same you would find in a Kindle. It runs on Linux and its "hard drive" is a 4 gigabyte or above SD card. It is powered by a normal cell phone charger or any other power supply that can give 5V and maybe 1A. You can program it to do certain things, like an Arduino or other microcontroller. You can program an Arduino from a Raspberry Pi, too.
How To Get Started
I won't be explaining every detail of the getting started process, but I will give some, like the parts and the SD card. First, you need to have: a Raspberry Pi, a power supply, a USB cable, a mouse, a keyboard, an SD card, a laptop, and a monitor. You need the laptop in order to format the SD card. If your laptop does not have an SD card reader, then you need to buy one that is compatible with your computer. Once you do that, go on to the Raspberry Pi website and download the Raspian file or other system, and, if you want to, download the torrent file. Next, move the file to the SD card, wait a little bit, and the operating system is now on the SD card. Furthermore, put the SD card in the Raspberry Pi, connect all the equipment to the Raspberry Pi, and power it on. If it does not work, check to make sure that everything is plugged in correctly, that you downloaded the system correctly, and, if all that checks out okay, pinch the SD card between your thumb and the Raspberry Pi board. Once you have it pinched, power on the board. If it works, find a way to keep the SD card tight. I used this strategy, and it worked. Check out the troubleshooting page on the Raspberry Pi website for more methods.
A really useful book I used was Getting Started with Raspberry Pi by Matt Richardson. It goes over a lot of stuff that I didn't include here. Once you have everything going, do some awesome projects with the Raspberry Pi, and have fun!