The Tribulation is a period of global catastrophe and war that is expected by Christians to occur during the Last Days. It is related to the Rapture and the Second Coming. It is expected to last seven years, and will end with the return of Christ to Earth.[1]
The Left Behind books and computer games and much Christian Survivalist fiction take up this theme.
See Book of Revelation and Historical exegesis of the Book of Revelation
The Tribulation is divided into two periods of 3.5 years. The first period is simply called the Tribulation. The second period is called the Great Tribulation.