The Year that Never Was was a year which never occured but was in the memories of many people who witnessed it. It was set betweeen May 2008 and 2009, and was when the Time Lord Empire was being created on Earth by the Master.
However, thanks to the Doctor, Martha Jones and Jack Harkness, the paradox machine used to subvert a temporal paradox thereby allowing the year to occur was destroyed, and so the events of the year did not actually happen. The only people with memories of the year were the Doctor, the Master, Jack Harkness, Lucy (the Master's companion) Martha Jones, Francine Jones, Clive Jones, Tish Jones, and (possibly) the soldiers aboard the Valiant. (DW: Last of the Time Lords)
