One of the most well-known ideas from the book of Revelation is that of the 144,000 who will have some important role in the future. John mentioned this number twice in conjunction with describing some special group from the tribe of Israel. The first reference is in Revelation 7, where he said, "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" (Revelation 7:4). He then went on to enumerate the fact that there would be 12,000 out of every tribe who were sealed: "Of the tribe of Juda were sealed twelve thousand. Of the tribe of Reuben were sealed twelve thousand...." Joseph Smith had questions about this group, and so He asked the Lord and recorded the answer in D&C 77. We read, "Q. What are we to understand by sealing the one hundred and forty-four thousand, out of all the tribes of Israel—twelve thousand out of every tribe? A. We are to understand that those who are sealed are high priests, ordained unto the holy order of God, to administer the everlasting gospel; for they are they who are ordained out of every nation, kindred, tongue, and people, by the angels to whom is given power over the nations of the earth, to bring as many as will come to the church of the Firstborn" (D&C 77:11). So from this we learn that this group will be missionaries who will bring people "to the church of the Firstborn," which we know from another revelation are those who inherit the celestial kingdom (see D&C 76:54).
John mentioned this group again when he wrote, "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" (Revelation 14:1). This is presumably referring to the time of the coming of the Savior when the He will stand on Mount Zion in the New Jerusalem (see D&C 84:2). John further described that this group of 144,000 would sing "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" (Revelation 14:3). If the next couple of verses are likewise talking about this same group, then we learn as well that they are "redeemed from among men" and are "without fault before the throne of God." The Lord appears to have quoted from these verses when He said in another revelation of the Doctrine and Covenants, "For behold, the Lord God hath sent forth the angel crying through the midst of heaven, saying: Prepare ye the way of the Lord, and make his paths straight, for the hour of his coming is nigh—When the Lamb shall stand upon Mount Zion, and with him a hundred and forty-four thousand, having his Father’s name written on their foreheads" (D&C 133:17-18).
Other than these scriptures we really have very little information about the 144,000. The most interesting commentary I could find came from Orson Pratt in which he suggested that the 144,000 would come at least in part from the lost ten tribes who will come down from the north and join with Zion at the New Jerusalem at some point before the Second Coming. Elder Pratt explained: “How long will they who come from the north countries tarry in the heights of Zion? Sometime. They have got to raise wheat, cultivate the grape, wine and oil, raise flocks and herds, and their souls will have to become as a watered garden. They will dwell in Zion a good while, and during that time, there will be twelve thousand chosen out of each of these ten tribes, besides twelve thousand that will be chosen from Judah, Joseph, and the remaining tribes, one hundred and forty-four thousand in all [see Revelation 7:4–8; D&C 77:11]. Chosen for what? To be sealed in their foreheads. For what purpose? So that the power of death and pestilence and plague that will go forth in those days sweeping over the nations of the earth will have no power over them. These parties who are sealed in their foreheads will go forth among all people, nations and tongues, and gather up and hunt out the house of Israel, wherever they are scattered, and bring as many as they possibly can into the Church of the first-born, preparatory to the great day of the coming of the Lord. One hundred and forty-four thousand missionaries! Quite a host. All this has got to take place.” So he suggested that the group of 144,000 will be formed after the lost ten tribes return, and at least 120,000 will be chosen from those tribes. It seems clear from all of these references that the 144,000 will be powerful missionaries because of their purity, and they will bring many souls to salvation from their labors.