The Raspberry Pi is a cheap underpowered computer that you can buy for $35 if you are willing to wait a little or about $46 -50 on Amazon and get it the next day. The specs of this small computer are pretty weak, the unit i’m currently using is the version B board and comes with 512mb ram, an ARM 700mhz GPU, network port, two usb ports, GPIO port, audio out port an HDMI port which carries both video and sound and an SD card slot for the bios and operating system. People use these devices in their personal projects to build arcades, home automation systems, robots and other interesting projects which require little processing power.
I got this unit to build a home automation system similar to the Belkin Weemo system but at a fraction of the cost. I plan to wire it up to a 8 relay board which will in turn control some wireless power switches.