Actor David Tennant, the 10th to play The Doctor since the show went on the air in 1963, noted in a recent interview that the 50th anniversary special "Day of the Doctor," which debuts worldwide simultaneously on November 23, will “move the legend forward in a fundamental way" by finally telling us what The Doctor did in the Time War. This cataclysm has been alluded to darkly many times since the show's return to the BBC in 2005 but not discussed in any great detail. In typical ret-conning "Who" fashion it contrives to fold into the canon the 16 year-hiatus that we here on earth experienced as just the show being off the air.
The mini-episode "Night of the Doctor" (below) answers some questions and raises a few more. The most glaring example is how these developments will effect the show's overall timeline and the numbering of Doctors. When John Hurt's Doctor was introduced at the end of the season just past he appeared to be some sort of Future Doctor or Last Doctor, The Doctor at the End of the Universe. Here we see Paul McGann's 8th Doctor regenerating into him. Worst case, all the later Doctors, from Christopher Eccleston's so-called 9th onward, would need to get bumped ahead and re-numbered. But I can't see that happening. What would they do with all the tee shirts? Stage a re-call? Offer free replacements?