Those people who adopt the Pre-Tribulation Rapture position believe that Jesus Christ will return for His children sometime before the Great Tribulation begins, as opposed to those who believe that He will return while the Great Tribulation is in progress, or immediately after it has ended. For reasons which are fully explained in his in-depth series,
The Great Tribulation And The Rapture, Bill Kochman currently believes that the Post-Tribulation Rapture is the most Scripturally-sound view of the various Rapture positions. Please read this in-depth series for a more-detailed explanation.