What is the Great White Throne Judgment?
The Great White Throne Judgment is the final judgment of the wicked dead, who are resurrected and judged according to their works, with anyone not found in the book of life cast into the lake of fire.
The Great White Throne Judgment takes place at the end of the Millennium. After Satan is loosed from the bottomless pit, deceives the nations, gathers them against Jerusalem, and is destroyed by fire from heaven and cast into the lake of fire, this judgment follows. Revelation 20:11 describes a great white throne with someone seated on it, before whose face the earth and heaven flee away, leaving no place for them.
This judgment is connected to the second resurrection, the resurrection of the wicked. Revelation 20:12 says the dead, both small and great, stand before God while the books are opened, along with the book of life, and the dead are judged according to what is written in the books, according to their works. Revelation 20:13 adds that the sea gives up its dead, and death and hell deliver up the dead that were in them, so that every person is judged according to their works.
Jesus holds the keys of hell and of death because of His resurrection, having been dead and now alive forevermore. At His own resurrection, He used these keys to release the righteous dead who had been held until the price for sin was paid, and they now dwell as the spirits of just men made perfect. At the Great White Throne Judgment, He uses these same keys again, this time to empty death and hell by resurrecting the wicked dead so they can stand before this judgment.
The outcome of the Great White Throne Judgment is final and eternal. Revelation 20:14 declares that death and hell themselves are cast into the lake of fire, calling this the second death. Revelation 20:15 states that whosoever is not found written in the book of life is cast into the lake of fire. This judgment stands as the last reckoning for the wicked before the new heaven and new earth are established.
The scriptures
And I saw a great white throne, and him that sat on it, from whose face the earth and the heaven fled away; and there was found no place for them. And I saw the dead, small and great, stand before God; and the books were opened: and another book was opened, which is the book of life: and the dead were judged out of those things which were written in the books, according to their works. And the sea gave up the dead which were in it; and death and hell delivered up the dead which were in them: and they were judged every man according to their works. And death and hell were cast into the lake of fire. This is the second death. And whosoever was not found written in the book of life was cast into the lake of fire.
And when the thousand years are expired, Satan shall be loosed out of his prison, And shall go out to deceive the nations which are in the four quarters of the earth, Gog and Magog, to gather them together to battle: the number of whom is as the sand of the sea. And they went up on the breadth of the earth, and compassed the camp of the saints about, and the beloved city: and fire came down from God out of heaven, and devoured them. And the devil that deceived them was cast into the lake of fire and brimstone, where the beast and the false prophet are, and shall be tormented day and night for ever and ever.
And when I saw him, I fell at his feet as dead. And he laid his right hand upon me, saying unto me, Fear not; I am the first and the last: I am he that liveth, and was dead; and, behold, I am alive for evermore, Amen; and have the keys of hell and of death.
Jesus, when he had cried again with a loud voice, yielded up the ghost. And, behold, the veil of the temple was rent in twain from the top to the bottom; and the earth did quake, and the rocks rent; And the graves were opened; and many bodies of the saints which slept arose, And came out of the graves after his resurrection, and went into the holy city, and appeared unto many.
Verily, verily, I say unto you, The hour is coming, and now is, when the dead shall hear the voice of the Son of God: and they that hear shall live.
Wherefore seeing we also are compassed about with so great a cloud of witnesses, let us lay aside every weight, and the sin which doth so easily beset us, and let us run with patience the race that is set before us,
To the general assembly and church of the firstborn, which are written in heaven, and to God the Judge of all, and to the spirits of just men made perfect,
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.