Gallifrey is the home world of the Time Lords. It is located in the constellation of Kasterborous, at galactic coordinates ten-zero-eleven-zero-zero by zero-two from galactic zero centre some 250 million light years away from Earth. It was destroyed during the Time War. The Doctor and The Master were the only known surviving sons of Gallifrey, until The Master was murdered while reigning over the Earth. The Time War is/was the conflict between the Time Lords, (the oldest race, masters of space and time travel) and the Daleks (a powerful race bent on universal conquest), resulting in their mutual destruction. It has been hinted that The Doctor had brought about the end. The Doctor has also referred to this conflict as "the last great Time War," implying that there had been others. Within the Time War conflicts can be fought through different periods in history and space as well as using time as a weapon itself. Preemptive strikes, time-loops, temporal paradoxes, and the reversal of historical events are examples of time's role through the war.
Time Lord Policy is one of non-intervention, but on a higher level, they protected the time vortex and kept the peace.
The Doctor is alone in the universe but frequents the Earth quite often for his fondness of man. One such member of the human race The Doctor has a particular affinity for was that of Rose Tyler. She was lost in another battle. He is alone again.
**Author's Note** these facts come from observations and fixed cannon, which can be researched further on outpostgallifrey.com or wikipedia.com. This is incomplete and will be updated soon.