The Raspberry Pi is a fully functional miniture computer. It's major appeal has been it's attractive price point (£25 - £30) matched with the Open Source based Raspian Operating System.
It allows for an easy way to start learning Linux. The Raspian OS itself is based on the popular Debian distribution, and offers an attractive desktop environment along with some education applications pre-installed.
With the Offical Desktop Startup Kit you can be up and running in next to no time at all. This comes with a pre-installed image of Raspian which can save some headaches if you have never had the experience of writing an OS to a MicroSD card.