Well, Harold Camping's predicted date for the rapture has come and gone, proving once more that the time for the end of the world is known only to God.
Yes, there is a large group of Christians who believe that one preliminary event to the Last Judgment is God's removal of Christians just prior to the Great Tribulation. I admit to be one who believes that God puts us here to face the various tribulations that hit, and overcome them, otherwise the Book of Revelation makes no sense.
Going on to explain the idea of the "rapture" to people without a grounding in Evangelical theology or lore, let me say...
Hey! My roof and ceiling are gone! I'm leaving my chair! I'm floating in the air! My bad sense of balance is throwing my heels over my head as I am floating upwards! I'm head down as I type! What the dickens is happening? Uh-oh! It's time for the Halleluuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuu