Who are the 144,000?
The 144,000 are sealed servants of God taken from all twelve tribes of Israel, described as young male virgins who follow the Lamb and are counted among the firstfruits redeemed to God.
Scripture identifies the 144,000 as a specific group sealed out of the twelve tribes of the children of Israel, with twelve thousand taken from each tribe: Juda, Reuben, Gad, Aser, Nephthalim, Manasses, Simeon, Levi, Issachar, Zabulon, Joseph, and Benjamin. They receive a seal of the living God in their foreheads, and this sealing must happen before the earth, sea, or trees can be harmed, showing that God protects His own before pouring out judgment.
These are described as young male virgins who are innocent and serve the Lord. Revelation 14 says they stand with the Lamb on Mount Sion, having the Father's name written in their foreheads, and that they sing a new song before the throne that only they can learn, because they are redeemed from the earth. They are called firstfruits unto God and to the Lamb, not defiled with women, following the Lamb wherever He goes, and found to be without guile or fault before the throne of God.
Their sealing takes place after the sixth seal is opened, while the Church is still on the earth, and before the wrath of God begins to fall. The order given is that the earth is not to be hurt until the 144,000 are sealed, showing they are marked out for God's special purpose and protection during that season before judgment is released.
At the sounding of the seventh trumpet, this sealed number, along with the great multitude of the redeemed, appears again in the record of those who worship before God's throne, standing as a testimony to God's faithfulness in preserving a remnant of Israel even in the midst of the coming tribulation.
The scriptures
And I saw another angel ascending from the east, having the seal of the living God: and he cried with a loud voice to the four angels, to whom it was given to hurt the earth and the sea, Saying, Hurt not the earth, neither the sea, nor the trees, till we have sealed the servants of our God in their foreheads. And I heard the number of them which were sealed: and there were sealed an hundred and forty and four thousand of all the tribes of the children of Israel. Of the tribe of Juda were sealed twelve thousand. Of the tribe of Reuben were sealed twelve thousand. Of the tribe of Gad were sealed twelve thousand. Of the tribe of Aser were sealed twelve thousand. Of the tribe of Nephthalim were sealed twelve thousand. Of the tribe of Manasses were sealed twelve thousand. Of the tribe of Simeon were sealed twelve thousand. Of the tribe of Levi were sealed twelve thousand. Of the tribe of Issachar were sealed twelve thousand. Of the tribe of Zabulon were sealed twelve thousand. Of the tribe of Joseph were sealed twelve thousand. Of the tribe of Benjamin were sealed twelve thousand.
And I looked, and, lo, a Lamb stood on the mount Sion, and with him an hundred forty and four thousand, having his Father’s name written in their foreheads. And I heard a voice from heaven, as the voice of many waters, and as the voice of a great thunder: and I heard the voice of harpers harping with their harps: And they sung as it were a new song before the throne, and before the four beasts, and the elders: and no man could learn that song but the hundred and forty and four thousand, which were redeemed from the earth. These are they which were not defiled with women; for they are virgins. These are they which follow the Lamb whithersoever he goeth. These were redeemed from among men, being the firstfruits unto God and to the Lamb. And in their mouth was found no guile: for they are without fault before the throne of God.
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.