Seven Year Tribulation
The Seven Year Tribulation is prophesied in Daniel chapter 9:
Seventy weeks have been decreed for your people and your holy city, to finish the wrongdoing, to make an end of sin, to make atonement for guilt, to bring in everlasting righteousness, to seal up vision and prophecy, and to anoint the Most Holy Place. - Daniel 9:24
This is a prophecy about Israel and the Jewish people. In this prophecy a "week" is a period of seven years. The 70 "weeks" are divided into three periods. One of seven "weeks" and another of 62 "weeks" and after 69 weeks the Messiah will come. Those 69 "weeks" are the time period that passed from when the prophecy was made until Jesus came. The third period is the 70th week that has not yet come. Since the crucifixion of Jesus Christ we have been in the "church age," an unspecified period of time between Daniel's 69th and 70th week. During the "church age" God is working through His church instead of Israel. Currently, the church, meaning all Christians, is God's representative on earth. However, that will end... possibly very soon.
The End of the Church Age
When the church age ends, and the 70th week begins, all Christians will be taken from the earth to make way for God's judgment to be poured out on the earth. The Latin word for "a carrying off" is used to describe what happens... rapture. During the church age it was the church's job to proclaim the gospel to the whole world (Matthew 28:19), but now that responsibility will return to the Jews.
The 70th Week
Daniel chapter 9 provides a brief description of the 70th week:
And he [the Antichrist] will confirm a covenant with the many for one week, but in the middle of the week he will put a stop to sacrifice and grain offering; and on the wing of abominations will come the one who makes desolate, until a complete destruction, one that is decreed, gushes forth on the one who makes desolate. - Daniel 9:27
What is this saying? The Book of Revelation gives us the details. To summarize: A world leader will rise up who appears to bring peace. He will rebuild the Temple in Jerusalem and the Jewish sacrifices will resume. After 3-1/2 years, that all ends... and the "beast will reign for 42 months (3-1/2 years -- Revelation 13:5), which is the time of the Great Tribulation.
For then there will be a great tribulation, such as has not occurred since the beginning of the world until now, nor ever will again. And if those days had not been cut short, no life would have been saved; but for the sake of the elect those days will be cut short. - Matthew 24:21-22
Notice how bad the Great Tribulation is. If it had not been limited to 3-1/2 years (cut short), no one would have survived. For the sake of the elect (those people who became Christians during the Tribulation times), God does not continue His judgment to the full extent that unbelieving people deserve.
The Seven Seal Scroll Judgment Period (Revelation 6 & 8)
The Seven Trumpet Judgment Period (Revelation 8-9)
During the second half of the Tribulation there will be two witnesses in Jerusalem preaching judgment and salvation. They are in addition to the 144,000 Jewish witnesses. These two men will have miraculous powers and no one will be able to kill them. At the end of the 3-1/2 years, when there is no more to say, they will be killed and the whole world will rejoice, celebrating and giving presents to each other. However, three and a half days later the two witnesses will be resurrected in the full view of the world, and will ascend to heaven.
The Seven Vial Judgment Period (Revelation 16-18)
Jesus Christ returns in great power and glory as the King of kings, Lord of lords. Satan and demons are thrown in the pit (lake of fire) and the unsaved will be taken to eternal punishment (outer darkness, Matthew 25:30 aka eternal fire, Matthew 25:41). Believers (the saved) who are are still living enter the thousand year reign of Jesus Christ on earth.
The following are links to additional information on other web sites:
What is the Rapture of the Church?
What is the Great Tribulation?
The Terrors of the Great Tribulation, Part 1
So all Israel will be saved; just as it is written: “The Deliverer will come from Zion, He will remove ungodliness from Jacob.” - Romans 11:26
All The Elect in Israel Will Be Saved
This verse is referring to the end of the Tribulation when all of those Jews who believe in Jesus Christ will go into the Millennial Kingdom.
The thousand year reign of Jesus Christ will be associated with Mount Zion:
The Lord will stretch out Your strong scepter from Zion, saying, “Rule in the midst of Your enemies.”. - Psalm 110:2
Jesus Christ is the Messiah... He is the Savior of both Jew and Gentile. Those who repent now, before the Tribulation, will not go through the Tribulation and they return with Christ to live with Him forever.
Those who do not repent until after the Tribulation starts, Jew and Gentile, will go through the Tribulation and most likely be martyred. They will return with Christ to live with Him forever. Those believers who do not die during the Tribulation will pass directly into the Thousand Year Reign of Christ on earth, and will live with Him forever in the peace and perfection that awaits all believers.