The Raspberry Pi is an ARM-based SoC (System on a Chip), an almost complete computer on a single, credit-card sized board. You only need to hook it up to standard peripherals to have a fully functioning machine. Its original purpose is to incite youth's interest in computing, but it has turned out to be a wonderful little cheap box for all kinds of hacking. You may order it online, but you can find it in stores too - e.g. in Luxembourg, Schaller Electronics has it available, including some interesting peripherals.
One possible use is as a small home server, obviously within limits due to its limited performance. It is this use that will be explored here.