The 144,000 in the Book of Revelation

144,000 persons are mentioned in The Book of Revelation (Rev 7:1–8). 
We have the following clues to identify these people and when they appear:

1. It says: "Do not harm the earth or the sea or the trees, until we have sealed the servants of our God on their foreheads." (Rev 7:3).
   It means that they are servants of God.
2. They are Jews represented by the twelve tribes of Israel mentioned in
   Rev 7:4–8. But the tribe of Dan isn't mentioned, see the end part
   of Man-Made Religion. This tribe is replaced by the Tribe of Manasseh.
   See also in Ezekiel's Temple why the tribe of Dan is mentioned there. 
3. These Jewish servants of God are "...sealed...on their foreheads" (Rev 7:3).
   Apostle Paul spoke of believers being "sealed" by the Holy Spirit (2 Cor 1:22, Eph 1:13, 4:30). This is common language for born again believers. This seal of the Lord is uniquely visible so that John can know without a doubt who these people are: Jews who had put their faith in Jesus Christ. 
4. The first verse of Rev 7 begins with the words: "After this..." The previous chapter describes the opening of the first six seals during the Tribulation period, see The Horses in the Book of Revelation. This means that these Jews receive their seals after the sixth seal is broken (Rev 6:12–17). Most likely they will become born again believers during the Tribulation period.

These Jews are servants of God who share the gospel of Jesus Christ with great signs and wonders during the Tribulation period. Many people are saved at this time. These Jews receive seals on their foreheads for protection against God's wrath (Rev 7:3), see God's Attributes. They will have the name of Jesus and God the Father written on their foreheads (Rev 14:1). They are protected from God's wrath but not from persecution. Many of them will loose their lives during the second half of the Tribulation period (Rev 14:2–5). There will be a great persecution of the Christians during this Great Tribulation (Rev 7:13–17, Mat 24:15–22, Mark 13:14–20).

People will be saved during the Tribulation period, through the Gentiles who are 
born again believers and through the 144,000 Jews, see The Two Witnesses.

The 144,000 Jews who have Jesus' name and his Father’s name written on their foreheads will stand with Jesus on the heavenly Mount Zion (Rev 14:1–5,
Heb 12:18, 22–23), see time 15:03–20:29 in Hebrews 12 (Part 4) :18-29.

When Jesus returns and frees Israel from its enemies at the end of the
Great Tribulation, the Jews will recognize him as Messiah, the One they had previously rejected, see The Depth of the Riches in The Book of Romans.

See also The Book of Revelation.