"Tribulation," according to Jesus, occurs as the result of one's faith in God and describes the persecution of that faith. It is distinct from other causes of persecution (i.e. racism, socio-economic class, etc.) and can happen at any time.

However, Jesus makes a distinction when speaking of tribulation in the context of the end of human history. Associated with Daniel's 70th week, when "the abomination of desolation which was spoken of through Daniel the prophet standing in the holy place," Jesus speaks of the "great tribulation" as an extreme and lethal persecution of Believers (Matt 24:29; Mark 13:24). While the Great Tribulation ends just before Jesus Christ returns at the seventh trumpet, Daniel's 70th week ends with the demise of the Antichrist (Rev 19:20-21) and "to make an end to sin" (Dan 9:24).

Because Jesus applies the term "tribulation" exclusively to Believers, He implies that Believers will be living through this period, and the tribulation ends with the demise of the last genuine Believer who would be persecuted for their faith and martyred.

The apostle John reports that this special class of Believers would be resurrected first and reign with Him for a thousand years before a new heaven and earth is ushered in. This special class of Believers are those who died because of their testimony of Jesus Christ and the word of God during the Great Tribulation, because they refused the mark of the Beast and did not worship him.

Then one of the elders answered, saying to me, "These who are clothed in the white robes, who are they, and where have they come from?" I said to him, "My lord, you know." And he said to me, "These are the ones who come out of the great tribulation, and they have washed their robes and made them white in the blood of the Lamb. (Rev 7:13-14)

Then I saw thrones, and they sat on them, and judgment was given to them. And I saw the souls of those who had been beheaded because of their testimony of Jesus and because of the word of God, and those who had not worshiped the beast or his image, and had not received the mark on their forehead and on their hand; and they came to life and reigned with Christ for a thousand years. The rest of the dead did not come to life until the thousand years were completed. This is the first resurrection. Blessed and holy is the one who has a part in the first resurrection; over these the second death has no power, but they will be priests of God and of Christ and will reign with Him for a thousand years. (Rev 20:4-6)

For deeper study:

What and When is the Tribulation?

What is the Great Tribulation?

Which heaven do Christians go to?