The Raspberry Pi is a cheap, credit-card sized computer. It runs a full linux operating system, so you can program in most any language that feels comfortable to you.
The Pi was built as a teaching tool, intended to get young people into programming by giving them a device that could truly be theirs. You can find more about the philosophy of the Raspberry Pi on the Raspberry Pi Foundation's Website
But what I think makes the Raspberry Pi special is its ability to interact with the real world. The Pi comes built with GPIO pins, which allow you to programmatically control exterior devices like a servo motor or an LED. Check out my page on Raspberry Pi Hardware Control to find more examples of this.