The Tribulation is a future time period when the Lord will accomplish at least two aspects of His plan: 1) He will complete His discipline of the nation Israel (Daniel 9:24), and 2) He will judge the unbelieving, godless inhabitants of the earth (Revelation 6 - 18). The length of the Tribulation is seven years. This is determined by an understanding of the seventy weeks of Daniel (Daniel 9:24-27; also see the article on the Tribulation). The Great Tribulation is the last half of the Tribulation period, three and one-half years in length. It is distinguished from the Tribulation period because the Beast, or Antichrist, will be revealed, and the wrath of God will greatly intensify during this time. Thus, it is important at this point to emphasize that the Tribulation and the Great Tribulation are not synonymous terms. Within eschatology (the study of future things), the Tribulation refers to the full seven-year period while the “Great Tribulation” refers to the second half of the Tribulation.
It is Christ Himself who used the phrase "Great Tribulation" with reference to the last half of the Tribulation. In Matthew 24:21, Jesus says, "For then there will be a great tribulation, such as has not occurred since the beginning of the world until now, nor ever shall." In this verse Jesus is referring to the event of Matthew 24:15, which describes the revealing of the abomination of desolation, the man also known as the Antichrist. Also, Jesus in Matthew 24:29-30 states, “Immediately after the tribulation of those days . . . the Son of Man will appear in the sky, and then all the tribes of the earth will mourn, and they will see the Son of Man coming on the clouds of the sky with power and great glory." In this passage, Jesus defines the Great Tribulation (v.21) as beginning with the revealing of the abomination of desolation (v.15) and ending with Christ's second coming (v.30).
Other passages that refer to the Great Tribulation are Daniel 12:1b, which says, "And there will be a time of distress such as never occurred since there was a nation until that time." It seems that Jesus was quoting this verse when He spoke the words recorded in Matthew 24:21. Also referring to the Great Tribulation is Jeremiah 30:7, "Alas! for that day is great, There is none like it; And it is the time of Jacob’s distress, But he will be saved from it." The phrase “Jacob’s distress” refers to the nation of Israel, which will experience persecution and natural disasters such as have never before been seen.
Considering the information Christ gave us in Matthew 24:15-30, it is easy to conclude that the beginning of the Great Tribulation has much to do with the abomination of desolation, an action of the Antichrist. In Daniel 9:26-27, we find that this man will make a "covenant" (a peace pact) with the world for seven years (one “week”; again, see the article on the Tribulation). Halfway through the seven-year period—"in the middle of the week"—we are told this man will break the covenant he made, stopping sacrifice and grain offering, which specifically refers to his actions in the rebuilt temple of the future. Revelation 13:1-10 gives even more detail concerning the Beast's actions, and just as important, it also verifies the length of time he will be in power. Revelation 13:5 says he will be in power for 42 months, which is three and one-half years, the length of the Great Tribulation.
Revelation offers us the most information about the Great Tribulation. From Revelation 13 when the Beast is revealed until Christ returns in Revelation 19, we are given a picture of God's wrath on the earth because of unbelief and rebellion (Revelation 16-18). It is also a picture of how God disciplines and at the same time protects His people Israel (Revelation 14:1-5) until He keeps His promise to Israel by establishing an earthly kingdom (Revelation 20:4-6).