The Great Tribulation refers to tumultuous events that are described during the "signs of the times", first mentioned by Jesus in the Olivet discourse. The Great Tribulation is also referenced in the Book of Revelation.
In the futurist view of Christian eschatology, the Tribulation is a relatively short period of time where anyone who chose not to follow God up until the Rapture and was therefore left behind (according to Pre-Tribulation doctrine, not Mid- or Post-Tribulation teaching) will experience worldwide hardships, disasters, famine, war, pain, and suffering, which will wipe out more than 75% of all life on the earth before the Second Coming takes place.
According to Dispensationalists who hold the futurist view, the Tribulation is thought to occur before the Second Coming of Jesus and during the End Times. Another version holds that it will last seven years in all, being the last of Daniel's prophecy of seventy weeks. This viewpoint was first made popular by John Nelson Darby in the 19th century and was recently popularized by Hal Lindsey in The Late Great Planet Earth. It is theorized that each week represents seven years, with the timetable beginning from Artaxerxes' order to rebuild the Temple in Jerusalem (the Second Temple). After seven plus 62 weeks, the prophecy says that the messiah will be "cut off", which is taken to correspond to the death of Christ. This is seen as creating a break of indeterminate length in the timeline, with one week remaining to be fulfilled.