The Raspberry Pi is a computer that is as big as a credit card. People call it a single-board computer because it comes with only one circuit board. This is not the only single-board computer. There are more of this kind of computer. The Raspberry Pi was created in the UK by the Raspberry Pi Foundation. Its purpose was to help high school students learn computer science. Lots of people use it to make things like cameras, gaming machines, and many more things. Some people make even more unusual things with it.
The Raspberry Pi does not come with an operating system by itself. You have to load either NOOBS (recommended), or load a Linux distribution that works with the Raspberry Pi (for example, Raspbian.)
↑NOOBS stands for New Out Of Box Software. It comes with Arch Linux ARM (very hard to use), OpenELEC (media center), Pidora (Fedora Linux remix), raspbmc (media center), Raspbian (recommended), and RISC OS (very hard to start using?).