It runs Linux but don't be afraid.
Using the lightweight, but powerful Linux, making the raspberry pi powerful and diverse. Many of today's popular operating systems use Linux (to an extent) as the core for their structure so it's easy to see why the Raspberry Pi Foundation chose to go with Linux as their main O/S. The foundation offers a Linux distro called Debian for it's users to download onto an SD card (SD card is used to boot the pi) and begin the installation process. Since Linux is in essence used to power many other O/S, the coding community has ported and coded other O/S to use on the raspberry pi. Most notably it can run Android 4.0, but in my eyes the real excitement is with the many version of the popular media center app called XBMC (Xbox Media Center).
- Popular Raspberry Pi O/S:
