The 144,000 people described in two passages of the Bible: Revelation 7:1-8 and 14:1-5. According to the first passage, they are "the servants of . . . God" (7:3) who are pictured as being sealed upon their foreheads. Verses 4-8 depict there being 12,000 sealed from each of the 12 tribes of Israel, giving a total of 144,000.
In Revelation 14:1 we are told that the seal on their foreheads is Christ's and his Father's name. The 144,000 are with Jesus, and they sing a unique song of worship, which no one else can learn (14:3). In 14:4-5 we are told more about their identity: "It is these who have not defiled themselves with women, for they are chaste [Greek, "virgins"]; it is these who follow the Lamb wherever he goes; these have been redeemed from mankind as first fruits for God and the Lamb, and in their mouth no lie was found, for they are spotless."
The Witnesses say these people represent the Church of Christ, the Israel of God, and this is a common view among Bible scholars, one that has had a long history in the interpretation of Revelation. The 144,000 are taken as a symbol of the Christian faithful. It is what Witnesses say next that is bizarre.
Supposedly God began collecting the 144,000 at Pentecost in A.D. 33 but stopped collecting them in 1935, when the full number was reached. Some of the 144,000 are still living, and they are all Jehovah's Witnesses. As of 1986 there were only about 9,000 of the 144,000 left alive. Since the Jehovah's Witnesses are a larger group, possessing several million members, how are the rest of the Witnesses regarded? As non-members of the body of Christ. They will still be saved in the end, but they will not be part of the body and will not go to heaven. They are considered part of the "great crowd" mentioned in Revelation 7:9-10.
In Jehovah's Witness "kingdom halls" (church buildings) only members of the 144,000 are allowed to receive communion when the Lord's Supper is celebrated (which is done infrequently). Sometimes kingdom halls celebrate the Lord's Supper and no one receives communion since none of the 144,000 are present.
Needless to say, the Witness's view is riddled with holes. When discussing the 144,000 with Witnesses, make them realize that they are being inconsistent in how they interpret the texts where the 144,000 are mentioned. They insist that there are exactly 144,000 of these people, no more and no less, making it a literal number, but they interpret every other detail about this group in a symbolic manner.
If they were consistent, they would have to conclude that the 144,000 are Jews, drawn equally from the twelve tribes of Israel (most of which are now unrecognizably lost), that they are all male virgins who do not lie and who have two names literally written on their foreheads. The problem is that almost none of the 9,000 Witnesses supposed to be members of this group fit this description. Many of them are married Gentile women with children, and they presumably have bent the truth more than a few times in their lives.
The only detail from the text that the Witnesses take literally is the number. Every other detail of the text is taken non-literally. They should either take all of the details literally or see the 144,000 as a symbol of the Church as a whole (as does virtually everyone else).