Originally there was only one operating system for this piece of technological marvelry however, that did not last too long. For just as there is no one way to bake an edible raspberry pi there is now no one way to run the electronic one.
The official sanctioned operating system for the Raspberry Pi is called Raspian. A derivitive of Debian, which was the original operating system, designed specifically for the Raspberry Pi and targeted for its original purpose. A device for educating students into the wonders of computer hacking.