The Moment, also known as the Galaxy Eater, was the most powerful weapon in the universe, which the Doctor aimed to use to end the Last Great Time War. It was the final creation of the Ancients of Gallifrey, and had an operating system so sophisticated that it became sentient. Until the Doctor stole it, it was locked away along with other forbidden weapons in the Time Vaults. (TV: The End of Time, TV: The Day of the Doctor)
The Tenth Doctor initially claimed that the Eighth Doctor created the Moment from a modified Key of Rassilon and De-mat Gun during the War. (COMIC: The Forgotten, Don't Step on the Grass) However, it was actually stolen by the War Doctor from the Time Lords, rather than constructed. (TV: The Day of the Doctor)
Initially, it was believed that the Doctor had used it to end the war, time-locking it in the process. (COMIC: Don't Step on the Grass) Supposedly, when the Doctor used the Moment, he destroyed Gallifrey, the Daleks, and the Time Lords. (COMIC: Sky Jacks) Thus, this is why the Ninth Doctor later stated that all the combatants of the Time War had burned and been "wiped out in one second". (TV: Dalek)
However, these memories were not entirely accurate. In reality, the Moment appeared to the War Doctor in the form of a face from his future, and gave him an opportunity to change the outcome of the Time War, by involving two of his future selves. Together, they were able to devise a way to freeze Gallifrey in time, while allowing the Daleks to destroy themselves in the crossfire. Thus, Gallifrey and the Daleks still vanished, and history was preserved, but the Doctor was ultimately able to prevent the genocide of his own people. The War Doctor and his future iterations retained those incorrect memories, until the timeline caught up with the last one involved, the Eleventh Doctor. (TV: The Day of the Doctor)