River Song's World was an alternate timeline created by River Song when she interfered with a fixed point in time, namely the death of the Eleventh Doctor. In this timeline, time itself had begun to unravel. All of Earth's history occurred simultaneously.
River Song prevented the "death" of the Eleventh Doctor, which occurred on 22 April 2011 at Lake Silencio, Utah. River, knowing that Madame Kovarian had forced her to assassinate the Doctor, drained her suit of power.
In this alternate timeline, the War of the Roses had entered its second year, pterodactyls flew through the air and Winston Churchill was the Emperor of the Holy Roman Empire and travelled on his personal mammoth. Charles Dickens was also in the middle of writing A Christmas Carol and was being interviewed by the BBC. River, with the aid of President Kennedy and Cleopatra, had transformed one of the Pyramids of Giza into Area 52. Earth was also experiencing the side effects of what were believed to be solar flares, but were actually messages from the original timeline, responding to a distress call sent out by River.
Churchill sensed that something was wrong because the time and date never changed. He had the Doctor, who was his soothsayer, brought to him and demanded he explain why the time never changed. After the Doctor had explained how this timeline had been created, Churchill and he were attacked by Silents, only to be rescued by soldiers led by the alternate Amy Pond. She remembered certain things about her travels with the Doctor in the original timeline, but initially couldn't remember that Rory Williams (one of her soldiers) was her husband.
This alternate timeline was ended when the Doctor convinced River Song to make physical contact with him in the form of a kiss. Because she and the Doctor were the opposite poles of the disruption in time, their touching caused time to revert to its original state, where River "killed" the Doctor. (TV: The Wedding of River Song)