Then I looked, and there before me was the Lamb, standing on Mount Zion, and with him 144,000 who had his name and his Father’s name written on their foreheads. 2 And I heard a sound from heaven like the roar of rushing waters and like a loud peal of thunder. The sound I heard was like that of harpists playing their harps. 3 And they sang a new song before the throne and before the four living creatures and the elders. No one could learn the song except the 144,000 who had been redeemed from the earth. 4 These are those who did not defile themselves with women, for they remained virgins. They follow the Lamb wherever he goes. They were purchased from among mankind and offered as firstfruits to God and the Lamb. 5 No lie was found in their mouths; they are blameless. (Revelation 14:1-6 NIV)
This passage above brings an interesting break in the action so far. In the previous chapter we have seen a description of the “beasts” that will rule over the world during the tribulation period. In this passage, however, we fast forward to the day that Christ will rule from Jerusalem. I say “fast forward” because we have not yet arrived at the point chronologically where Christ comes down to Earth and stars ruling from Jerusalem (see Revelation 20:1-6). Nevertheless, the kingdom has already been proclaimed in heaven, foreshadowing that Christ will conquer Earth very soon (Revelation 11:15). Therefore we see that the conquest and redemption of the Earth is seen virtually as an established act, since nothing can defeat the sovereign purposes of God. We should understand this part of the vision, then, as being a glimpse ahead of time of what will take place in Revelation chapter 20.
So now we bring ourselves to the big question: who are the 144,000 people standing on Mount Zion (i.e. Jerusalem) with the name of God the Father and God the Son written on their foreheads? The most likely answer is that these are the 144,000 Christ-believing Jews* that will be sealed during the tribulation period (see Revelation 7:4). They will have a special relationship with Christ. It could be that they survive the entire tribulation period in order to rule with Christ, or they could be ressurrected with new bodies. Whatever the case they are considered “virgins” apparrantly because they are so dedicated to God that they practice celibacy.
*These 144,000 do not necessarily have to be Jews though. An alternative interpretation is that they could be any of the “servants of God” sealed in Revelation 7:2-3, not necessarily the 144,000 Jews. Indeed, it seems unlikely that all 144,000 believing Jews that are sealed during the tribulation will be celibate.