The Tribulation is a seven year long period of time (these are prophetical years of 360 days, not the 365.25 day long solar years we currently observe) spread across two 3 1/2 year time periods in which evil will run rampant on the Earth without restraint or restriction.  It will be a time when Satan will be allowed to do pretty much anything he wants (his power is not totally unchecked as he's somewhat restrained, but only slightly so) during which he will setup a one world empire, a one world economy and currency, and a one world religion focused on the worship of himself.  During this time anyone who resists either himself or his AntiChrist, especially Tribulation saints, will be executed.

It will be a time of great darkness, evil and trouble unlike anything seen before in history, or will ever be seen again.  This is the time known in the bible as "The Time Of Jacob's Trouble".  During or around this time six major wars will occur that will, in majority part, kill off between 90% and 98% of the world's population in the time leading up to the return of the Lord Jesus at His second coming.  This time period begins with a peace treaty and ends with the Glorious Appearing of Christ.  To learn more, consider searching for other prophecies related to this one all occurring during this time period.  Along with all of these other previously listed events are twenty one judgments against Satan and his kingdom.

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(*Do you have a better description, or did you find an error? Please let me know. Thanks*)

The Coming Great Tribulation (Billy Crone) - Report Link
How Bad Will the Tribulation Be? (Dr. David Jeremiah) - Report Link
The Seven-Year Tribulation: Is it, or Isn’t It? (Gary Stearman) - Report Link
The Purpose For The Tribulation (Alice Childs) - Report Link
What are the seventy weeks of Daniel? (GotQuestions.org) - Report Link
The Wars of the End Times (Dr David Reagan) - Report Link