Doomsday cults are (generally) religious movements whose teachings have an unhealthy focus on an apocalyptic event — anything from a local disaster to earth’s final destruction.
This event is referred to — either by the group or by others — as ‘Doomsday’ (Day of doom, End Time, End of Times, Last Days, etcetera), and is generally seen as a final judgment or the prelude to a final judgment in which a select group of believers is saved.
The teachings of doomsday cults usually include prophecies regarding the End Times.
Often a fail-safe option is built in. For instance, if the prophecy did not come to pass and the world did not end, the cult’s prophet/leader may claim that God changed the date in order to allow more people to be saved, that the calculations were off because not everything had yet been revealed, or the prayers of the faithful were answered by postponing the end.
Some doomsday cults are prepared to use violence in an effort to trigger the apocalypse (e.g. Aum Shinrikyo).
Others at some point encourage or force followers to commit suicide, ostensibly in preparation for a “better life.” (e.g. Heaven’s Gate, Movement for the Restoration of the Ten Commandments of God).
Not all who believe in doomsday belong to organized cults or other religious movements. See, for instance, For rent: a front-row view of the Apocalypse (said to take place on December 21, 2012).