... Revelation 7 first looks to the salvation of those in Israel who remain true to the Messiah despite persecution. The number 144,000 is surely symbolic. But these from Israel are not alone. There is a great multitude there as well from every nation, tribe, people and language (7:9). Who are they? They are all those from all the nations who have remained true to God throughout times of great tribulation and persecution by those who oppose Christ.
In the past, some have seen this gathering in God's throne room as a result of a “rapture,” the snatching of the faithful from the earth before the final judgment. They would connect this passage to 1 Thessalonians 4:17, where “we who are still alive and are left” are snatched up to meet Jesus in the clouds. There are some similarities to 1 Thessalonians 4 but also some differences.
The biggest similarity is that John seems to picture individuals who are alive when the final judgment begins. The great multitude of Revelation 7:14 has come out of the great distress and tribulation that John is saying will soon take place. They are not said to be all the saints of all the ages. To be clear, they do not come out of a seven year Tribulation, but Revelation seems to see them as those from the earth who are saved in the time leading up to the final judgment.
The big difference between Revelation and 1 Thessalonians 4 is that, in Thessalonians, the resurrection of the dead in Christ happens before this snatching from the earth. Further, it seems like those who are "caught up" to meet Christ in the clouds in Paul's understanding then return to participate in the judgment of the world (cf. 1 Corinthians 6:2-3). By contrast, those in Revelation 7 are not said to participate in the final judgment.
We will also notice below that the book of Revelation pictures two resurrections separated by a symbolic thousand year period. Since the book of Revelation is so symbolic, it would be perilous to use it to provide the basic framework for how we think the events will specifically play out. Better to stick with Paul's sense that the resurrection and evacuation/salvation of those in Christ will take place together before the final judgment of the earth. 
 N. T. Wright is well known for applying his genius to 1 Thessalonians 4 so that it might be taken a little less literally. See, for example, Paul for Everyone: Galatians and 1 Thessalonians (London: SPCK, 2002), 124-26. While he is surely correct that Paul is not picturing the saints going off to heaven forever, while he is surely correct that the image is more of meeting a visitor to escort them back into the city, he may be guilty of trying to "demythologize" the imagery a little too much. Paul probably did picture Jesus as straight up and descending straight down. Wright may certainly be correct that heaven is in another dimension, but no biblical author was equipped to understand what that statement even means.