Immediately on death the particular judgment is passed by Christ on the moral state of a person's soul. In Hebrews 9:27 St. Paul says, "[It] is appointed unto men to die once and after this comes the judgment." This judgment will be rendered privately by Christ to each person without any witnesses. In Romans 14:12 St. Paul teaches, "Therefore every one of us will render an account to God for himself."
At death a person's soul separates from his body and goes alone before Christ to be judged. At that moment Christ will welcome the perfect directly into Heaven, send the just, who still need purification, temporarily into Purgatory, or send the reprobate, those who die in mortal sin, immediately into Hell for all eternity.
Paragraph 1022 of the Catechism of the Catholic Church (CCC) explains, "Each man receives his eternal retribution in his immortal soul at the very moment of his death, in a particular judgment that refers his life to Christ: either entrance into the blessedness of heaven through a purification or immediately, or immediate and everlasting damnation."
At that moment a person's entire life, everything he's done or failed to do, will be judged; this includes his every thought, word and deed. In Matthew 12:36 Our Lord warns, "I say unto you, that every idle word that men shall speak, they shall render an account for it in the day of judgment." The particular judgment will be final and irrevocable. It will be repeated in front of all at the general judgment after the resurrection of the dead.
Paragraph 1022 of the CCC also quotes Doctor of the Church St. John of the Cross: "At the evening of life, we shall be judged on our love." Christ's judgment will not be based on how much He has infinitely and unconditionally loved us. He will, rather, judge us on how much we have loved Him. In John 14:15 Christ defined love: "If you love me, keep my commandments."