I could write pages of info on this little device, but others have done that, so I am pointing to some links.
http://www.raspberrypi.org/about and this http://elinux.org/RPi_Hub
If you enjoy, or want to learn about micro electronics, this is the device to play with. It is the premium techie toy that all can love. It has kept me amused for many months
The operating system is `Raspian' and has the (linux) lxde for a graphical front end. If you are familiar with Linux, then the Pi device is the same in many ways.
After the original boot up from the SD card, do `apt-get install synaptic' and you can then select what software to install for your use. The device can be set to auto boot to a graphic environment, but it is better to have it boot to a command line login, and then type `startx' to boot the graphical front end.
What you do with the device is up to your creativity. Because the operating system lives on an SDHC card, then you can have multiple SD cards with the OS installed in different ways (as a boot image), and then choose the SD card that you want to use. This being done from a tool concept. Using `apt-get install gnome-system-tools' will give you android on reboot as example.
The device can be powered from a switched USB hub. The power does need to supply a constant amp rate or strange things can/will occur.
This fellow has a website running on a Pi device http://www.arrowsmith.net.au/