What exactly is Rapture?
Nobody knows.
We only can tell from the information left us in the Bible combined with today's current events. Theologians have done incredible amounts of work deciphering what Our Lord's Rapture (being caught up) really is.
Will a good number of people be taken all at the same time? Yes. Will it all be at once? Probably. Will it be everyone who believes in Jesus? Maybe, but that sounds too easy. Most believe you'll have to have followed Jesus's ways here on earth first.
Will all others be doomed? No, but they will at least witness some real hell on earth first. Elijah and Moses will take many believers after three and one half years into Tribulation, and most scholars believe the Lord will take people all along the Tribulation period as they turn to him.
The two books of Thessalonians explains quite a bit about Rapture. It is mentioned in Matthew and John and is talked about in 1 Corinthians. Many Old Testament books mention it. Revelation ch. 3 states that it will clearly happen to the Philadelphia type congregations. The event happens sometime before the first of the seven seals are broken in Rev. 6. If you are interested in Rapture make sure you read the study links as most of the information is there or it will tell you where to find it.
How will we be taken? Most scholars say we will be flesh alive and vanish to Heaven or to a Heavenly holding place for seven years. Some suggest we won't have to be flesh alive, We still of course will be truly alive into the Heaven. But we might appear dead on earth. It could look like a virus spread quickly etc. It will be a lot of people at once. It will be like a flash when it transpires. There will be no suffering, for that is the whole meaning of Rapture. The Lord sweeps up his loved ones before the "hell" or Tribulation starts.
Read Revelation. Read our study. You will understand what God expects of you. Practice it. You will be his chosen to be included in Rapture.