Put simply, the Rapture is the moment that all true believers in Jesus Christ as their Lord and Savior are removed from this earth by the Lord Himself.
The Rapture will take place on a specific, predetermined date and time known only by God.
Our best guess can only come from specific clues given in Scripture by Jesus.
These are signs by which the Lord tells us to look up and be ready when we see them.
In the past, such signs have been used by many to herald the Lord's return, only for them to look foolish for trying to predict something based on 20% of the facts needed.
When this will take place in history, (during or after the tribulation), is very controversial among Biblical Christians. Despite differences of opinions on this subject, we should remember that such minor disagreements can not and should not deter us from our goal of preaching the Gospel, which is superior to such debates. The administrators of this site hold to the doctrine of Post-Trib, Pre-Wrath which says that the tribulation is the period of time we are now in, and will culminate with the Rapture, followed by God pouring out his wrath on an unrepentive earth.
We believe this generation is the only generation in history that has seen or will see, (due to technology), all the signs that Jesus talks about in scripture regarding His coming. This is also part of the last generation, as described in Matthew chapter 24 which was also alive at the time that Israel became a nation in 1948. Which means that according to the biblical definition of a generation, not all shall pass away from this generation until the Lord returns. It has been 68 of our years since the rebirth of Israel. Right now, the oldest living human is documented at age 111. If someone born in 1948 lives to be that old or older thanks to modern medicine, than we have up to (approx.) 2059 (give or take a few more years). So, Biblically, the rapture will occur between now and (approx.) 2059.
So we all look up in anticipation, waiting for the appointed time.