Revelation 20 describes a thousand-year reign of Christ with the saints, and the timing of that millennium relative to Christ's return has produced the four positions that organise the subject. They cut across denominations rather than defining them, and each has serious exegetical defenders.
Amillennialism holds that the thousand years is not a future period at all but a symbol of the present age between the resurrection and the return, during which Christ reigns and the departed saints live with him. Augustine set the pattern in the City of God by reading the millennium as the era of the church, and this has been the majority position of Roman Catholicism, Eastern Orthodoxy, and the Lutheran and Reformed confessions ever since.
Postmillennialism holds that Christ returns after a long period in which the spread and success of the gospel produces a golden age. It was widespread in Reformed and evangelical thought in the eighteenth and nineteenth centuries, where it supplied much of the energy behind missionary expansion and social reform, and it declined sharply after the First World War, though it has been revived in some Reformed circles since.
Historic premillennialism holds that Christ returns before the millennium and then reigns on earth. It has genuine patristic advocates in Justin Martyr, Irenaeus and Tertullian, was displaced by the Augustinian reading, and has modern defenders, George Eldon Ladd among the most careful.
Dispensational premillennialism was developed by John Nelson Darby in the 1830s and spread widely by the Scofield Reference Bible from 1909. It distinguishes God's programmes for Israel and for the church, and adds a rapture of the church before a seven-year tribulation that precedes Christ's return in glory. It became dominant in much of American evangelicalism and fundamentalism, drives a very large popular literature, and has had measurable political consequences in shaping attitudes toward the modern state of Israel and toward international affairs. It is much the most recent of the four; its critics regard it as a nineteenth-century novelty imposed on the text, and its defenders as the recovery of a consistently literal reading that allegorising had suppressed.