low-power, credit-card-sized computer
In their early days, computers were the size of entire rooms. Yet the power of a room-sized computer back then pales in comparison to the power of a mobile phone today! The Raspberry Pi (RPi) is a small, single-board computer that is similar to the hardware in a mobile phone. It is energy-efficient like a phone — no need for cooling fans! And, while the RPi has plenty of input/output ports (HDMI, audio, network, USB), many users connect only network and power cables, operating it as a headless computer.
Raspberry Pi's are great for non-intensive background computing. A single RPi can easily run a media player OS, a home automation setup, or an NFS fileserver. Headless RPi's accessed via SSH work well with lightweight, text-based software like the cmus music player, the Lynx web browser, and the WeeChat IRC client.
The Raspberry Pi was created by the non-profit Raspberry Pi Foundation as a cost-effective way to expose students to programming. An RPi with case and power cord can be purchased online for ~$50. The Foundation also oversees the development of Raspbian, a version of Linux tailored especially for the RPi. In addition, there are many Raspberry Pi hardware/software variations and add-ons supported by a large community of teachers, learners, and enthusiasts.