Donna's World

Donna's World was a rewritten version of the timeline, created when Donna Noble turned right instead of left while deciding which of two different job interviews to attend. As a result, the Tenth Doctor never met Donna and died underneath the Thames on Christmas Eve 2007. This meant that he also never met Martha Jones. These events resulted in devastating effects for Earth and the universe. The Doctor's former companion, Rose Tyler, along with the alternate timeline version of Donna, eventually restored the original timeline.

History
While on the planet Shan Shen, Donna was unknowingly fitted with a time beetle, part of the Trickster's Brigade. A fortune teller selected a point in her life where making a seemingly random choice had enormous ramifications.

On 25 June 2007, Donna had to make a choice between two jobs. The moment she chose between the jobs was at a junction. Her agency had offered her a contract with H.C. Clements, which while temporary, was posh and in the centre of London, which Donna enjoyed. Her mother, Sylvia Noble, had got her a job opportunity to become the personal secretary of Jival Chowdry, owner of a photocopying business. Although this would be a permanent job, Donna found it less interesting.

While driving and in discussing the dilemma, Sylvia pressured her into turning right to stop by Jival Chowdry for a chat. While in the regular timeline Donna ignored her mother and turned left; in this timeline she turned right, resulting in her taking the PA job. (TV: Turn Left)

Christmas 2007
The Tenth Doctor tracked down the Empress of the Racnoss to the Torchwood base underneath the Thames Flood Barrier and blew it up, causing the Thames to be drained into the abandoned Torchwood base. (TV: The Runaway Bride) Although he succeeded in defeating and drowning the Racnoss and its offspring, the Doctor drowned with them. For some reason, he didn't regenerate. It was specluted that he drowned too fast to regenerate. His body was retrieved by UNIT soldiers and his TARDIS was salvaged from under the Thames, later being sent to a UNIT base in Leeds. Donna stumbled across Rose Tyler, who arrived on the scene too late to do anything. (TV: Turn Left)

2008
The Judoon H2O scooped the Royal Hope Hospital to the Moon to find a Plasmavore criminal and her Slab accomplices. Although they found and killed the alien criminals, the Plasmavore had already armed the MRI. (TV: Smith and Jones) Sarah Jane Smith, Luke Smith, Maria Jackson, and Clyde Langer, who had been in the hospital when it was transported, disarmed it, but they died, along with the rest of the hospital, when their oxygen ran out. The only survivor was Oliver Morgenstern, who was passed the last oxygen tank by medical student Martha Jones, who gave up her life to save him. (TV: Turn Left)

Rose Tyler visited Donna and advised her to persuade her family to spend Christmas some place other than London. (TV: Turn Left)

Christmas 2008
A replica of the Titanic crash-landed onto Buckingham Palace. It destroyed the Greater London Area and flooded the whole of Southern England with radiation. Donna's family, who did decide not to stay in London for the holidays, having won a luxury getaway in a raffle, watched the events on television and were able to see the mushroom cloud of the explosion from a distance. (TV: Turn Left)

2009
Britain went under martial law in order to deal with the crisis caused by the nuclear explosion. Many citizens fled to France, until France closed its borders. The United States promised fifty billion pounds in financial help for Great Britain but this plan was abandoned when sixty million Americans were fully converted into Adipose children. (TV: Turn Left) Miss Foster's (TV: Partners in Crime) plan was not only successful, but sixty times as successful as was planned in the original timeline. (TV: Turn Left)

The Sontarans activated the ATMOS devices in the cars to convert Earth into a breeding ground. (TV: The Sontaran Stratagem / The Poison Sky) The surviving members of the Torchwood 3 team worked to stop the scheme. The team destroyed the Sontaran flagship and successfully reverted the atmosphere to its previous state, but Gwen Cooper and Ianto Jones died in the attempt; Jack Harkness, being immortal, survived and was captured by the Sontarans and transported to Sontar as a prisoner. Britain was largely unaffected due to its poverty and hence its lack of vehicles with ATMOS devices. Rose Tyler once more appeared to Donna to try to persuade her to help change back the timeline. (TV: Turn Left)

Roughly three weeks after the Sontaran invasion, the British government made a new law ("England for the English") forcing all non-British citizens in Britain to go to "labour camps". Donna's grandfather lamented that "he never thought he would see this happen again", and when Donna chided him, saying that they are just relocation camps, he replies that "that's what they said last time". Soon afterwards, the stars began going out. (TV: Turn Left)

Rose Tyler, through methods unknown, was working with UNIT on reverse-engineering the TARDIS, creating a time machine with which Rose hoped to restore the timeline. (TV: Turn Left)

Negation
UNIT and Rose Tyler sent Donna back to the point where the timelines diverged. According to Rose, in order to restore the timeline, Donna had to make her past self turn left instead of right at the point where she decided whether to follow her mother's advice or her own instincts. Donna arrived in 2007, but too far away from her past self to directly influence her; instead, she orchestrated an accident by stepping out in front of a lorry, which blocked traffic and forced the earlier Donna to turn left. Just before her death, Rose Tyler appeared and whispers a message for her to give the Doctor when the timeline was fully restored. (TV: Turn Left)

Behind the scenes

 * It is not actually a parallel world since the walls between realities were sealed in TV: Doomsday. This was made explicit by the production team. Rather it is an altered version of the Doctor's world. According to quantum physics, every decision we make creates multiple realities - one for each option of that decision. This theory implies that, despite the timeline being restored, the Donna's World reality still continued to exist in some form.
 * David Tennant did not actually play the person lying on the stretcher in the scene where they pull the body from the Thames.