In a moment, in the blink of an eye, every single Christian in the world will be caught up to heaven (Raptured) right before the Tribulation begins. While the Tribulation won't begin immediately after the rapture, there will be a period of time that will occur between the two events, even though its total duration is uncertain. Even so, it's well agreed upon that it won't be a long time. A few months to a few years at most. The purpose of the rapture and its timing within the confines of history is two fold. The first is to remove the Holy Spirit (aka The Restrainer) from the Earth to make way for a number of soon coming prophetical events, including the rise of the AntiChrist.
The second purpose of this event is to rescue the church and remove them from the Earth right before the Tribulation starts in order to protect them from "The Day of the Lord", a period of incredible wrath, torment and suffering that will soon befall the whole world. This is the event of which "No man knows the day or the hour" of its occurrence. There's considerable speculation on when this event will actually occur, such as on the Feast of Trumpets. However, as the bible saysm, nobody will know the day or the hour this will happen. There's also speculation, with good real world backing, that when the rapture happens, it will be explained away in numerous possible ways, including that all the "bad people" were destroyed by God, or even crazier, that all believers were snatched away in a world wide alien abduction.
(*Do you have a better description, or did you find an error? Please let me know.