It was stated in one of the doctor who books, based on the travels of the 8th doctor. ' The ancestor cell', that the doctor tried to prevent the 'faction paradox' from starting a war between the time lords and an unamed enemy but, in doing so, caused the destruction of Gallifrey.
In another book, (same series of books) it was believed that Gallifrey had been blown up but the Time Lords had survived but it became impossible to access any part of Gallifrey's history or future.
In the new doctor who series, the doctor says he is the last of the Time Lords. The Time war (to date) hasn't been shown but many references have been made by the 9th Doctor. What we know is…
! The Time Wars was fought between the Time Lords and the Daleks, (the doctor fought in this war), the Daleks, were apparantly beaten but only at the expense of the entire Time Lord race (except for the doctor).
! You learn that only the Doctor and the Dalek seen in 'Dalek', survived but after the Dalek commited suicide, the doctor believed he was the last remaining survivor.
The Time War was felt throughout time and space and many of the aliens you see in the first season encountered problems due to it.
! You find out however that the Dalek emperor's ship survived but fell through time, they lay hidden and slowly rebuilt until finally showing themselves in the season 1 finale.
That's all I can be arsed to explain right now…except for....
! The Time War is believed what caused the doctor's regeneration of the to the 9th doctor. I should of just said..
No, you haven't seen it