What is the Great Tribulation?
The Great Tribulation is the final three and a half years before Christ's return, when the Antichrist breaks his covenant with Israel, sets up the abomination of desolation in the temple, and unleashes fierce persecution against both Israel and the Church, distinct from the later outpouring of God's wrath.
The Great Tribulation is described as the last three and a half years of a seven year period, following what Scripture calls "the beginning of sorrows." It begins in the middle of a seven year covenant the Antichrist makes with Israel. For the first half of that covenant he acts as Israel's apparent deliverer, allowing the temple to be rebuilt and sacrifices to resume. But in the midst of the week, as Daniel 9:27 says, "he shall cause the sacrifice and the oblation to cease, and for the overspreading of abominations he shall make desolate." He goes into the temple, sits on the mercy seat, and demands worship as God. This is the abomination of desolation Jesus warned of in Matthew 24:15, and Daniel 11:31 describes armies polluting the sanctuary and taking away the daily sacrifice to set up this idol.
When Israel refuses to worship him, the Antichrist turns in fury against Jerusalem, and this launches the Great Tribulation itself, a time of persecution unlike any before it. Matthew 24:21 calls it "great tribulation, such as was not since the beginning of the world to this time, no, nor ever shall be." It is not only the Jews who suffer; the Church is on the earth during this reign and is hated and afflicted for Christ's name. Revelation 13:7-10 shows the beast making war with the saints and overcoming them, with power given him over every nation, tongue, and people. Believers are called to patience and faith in the middle of this, refusing the mark of the beast even at the cost of martyrdom, as pictured in the souls under the altar in Revelation 6:9-11, the faithful remnant who will not submit.
During this same window, many will be offended, betray one another, and grow cold in love, while false prophets rise and deceive many, and iniquity abounds. Yet those who endure to the end are promised salvation. Believers in Judaea are told to flee to the mountains when they see the abomination set up, because the danger and distress will be so severe that, apart from divine shortening of those days, no flesh would be saved.
It is important to distinguish the Great Tribulation from the Wrath of God, even though both occur near the end. The tribulation is the trying and persecution of the saints through affliction, while the wrath is God's vengeance poured out on the wicked, described as being contained in the seven last plagues of Revelation 15:1. Paul makes clear that "God hath not appointed us to wrath, but to obtain salvation by our Lord Jesus Christ" (1 Thessalonians 5:9), showing that the Church, though it passes through great tribulation and persecution at the hand of the Antichrist, is not the object of God's wrath, which falls afterward on an unrepentant world.
The scriptures
Then shall they deliver you up to be afflicted, and shall kill you: and ye shall be hated of all nations for my name’s sake. And then shall many be offended, and shall betray one another, and shall hate one another. And many false prophets shall rise, and shall deceive many. And because iniquity shall abound, the love of many shall wax cold. But he that shall endure unto the end, the same shall be saved. And this gospel of the kingdom shall be preached in all the world for a witness unto all nations; and then shall the end come.
When ye therefore shall see the abomination of desolation, spoken of by Daniel the prophet, stand in the holy place, (whoso readeth, let him understand:) Then let them which be in Judaea flee into the mountains: Let him which is on the housetop not come down to take any thing out of his house: Neither let him which is in the field return back to take his clothes. And woe unto them that are with child, and to them that give suck in those days! But pray ye that your flight be not in the winter, neither on the sabbath day: For then shall be great tribulation, such as was not since the beginning of the world to this time, no, nor ever shall be. And except those days should be shortened, there should no flesh be saved: but for the elect’s sake those days shall be shortened. Then if any man shall say unto you, Lo, here is Christ, or there; believe it not. For there shall arise false Christs, and false prophets, and shall shew great signs and wonders; insomuch that, if it were possible, they shall deceive the very elect. Behold, I have told you before. Wherefore if they shall say unto you, Behold, he is in the desert; go not forth: behold, he is in the secret chambers; believe it not. For as the lightning cometh out of the east, and shineth even unto the west; so shall also the coming of the Son of man be. For wheresoever the carcase is, there will the eagles be gathered together.
And he shall confirm the covenant with many for one week: and in the midst of the week he shall cause the sacrifice and the oblation to cease, and for the overspreading of abominations he shall make it desolate, even until the consummation, and that determined shall be poured upon the desolate.
And arms shall stand on his part, and they shall pollute the sanctuary of strength, and shall take away the daily sacrifice, and they shall place the abomination that maketh desolate.
And when he had opened the fifth seal, I saw under the altar the souls of them that were slain for the word of God, and for the testimony which they held: And they cried with a loud voice, saying, How long, O Lord, holy and true, dost thou not judge and avenge our blood on them that dwell on the earth? And white robes were given unto every one of them; and it was said unto them, that they should rest yet for a little season, until their fellowservants also and their brethren, that should be killed as they were, should be fulfilled.
And it was given unto him to make war with the saints, and to overcome them: and power was given him over all kindreds, and tongues, and nations. And all that dwell upon the earth shall worship him, whose names are not written in the book of life of the Lamb slain from the foundation of the world. If any man have an ear, let him hear. He that leadeth into captivity shall go into captivity: he that killeth with the sword must be killed with the sword. Here is the patience and the faith of the saints.
And I saw another sign in heaven, great and marvellous, seven angels having the seven last plagues; for in them is filled up the wrath of God.
For God hath not appointed us to wrath, but to obtain salvation by our Lord Jesus Christ,
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.