Soul

A part of a human being’s spiritual identity which is eternal and leaves the body upon death to go to the afterlife.

• Tampering with the soul is to break the "deepest laws of magic" according to Dumbledore (DH35).
• When a person commits murder, his or her soul is torn and can only be repaired by remorse (DH6)
• Voldemort's soul was split into six parts intentionally by using Dark Magic to create a series of Horcruxes after tearing his soul committing murder. The remaining part of the soul that remained as his identity could not be killed off without destroying the other six, giving him a type of immortality so long as he could possess a body or be reborn inside his own (PS17, CS17, GF32, GF33, OP36).
• The only way to kill Voldemort was to destroy the Horcruxes one by one until they were gone (HBP23).
• When Voldemort attacked Harry as a baby, the blood protection from his mother Lily's sacrifice caused the Killing Curse to rebound back to Voldemort killing him instantly. But because his soul was anchored to earth by the Horcruxes and could not move to the afterlife, Voldemort's soul was unstable and shattered, embedding part of itself into Harry's forehead and becoming his famous lightning-shaped scar (PS1, DH33, DH35). Thus Harry became in Dumbledore's word "the seventh Horcrux ...the Horcrux he never meant to make" (DH35).
• Harry's soul was whole and unblemished, but he shared a body with a soul-bit of Voldemort, a state of being which Dumbledore's silver instrument described as "in essence divided" (OP22). That is why Harry could sometimes view things from Voldemort's point of view, or even Nagini the snake, since the creature also contained a Horcrux (OP21, OP24).
• The healthy state of Harry's soul kept Voldemort from trying to possess him again after the attempt at the Ministry of Magic because it was too painful for Voldemort's tortured maimed soul to share a body with Harry's complete soul, as Dumbledore explained to Snape in a Pensieve memory (DH33). Just after the possession, Dumbledore explained it to Harry as love protecting him, perhaps meaning that his soul was where his love resided (OP22).
• The soul-bit finally left Harry when he confronted Voldemort in the Forbidden Forest after the Battle of Hogwarts (DH34, DH35). Because Harry marched to his death and willingly sacrificed himself without a fight, this time Voldemort's Killing Curse removed the Horcrux soul-bit and sent it alone to the afterlife, which Harry viewed in his vision of King's Cross as a weak deformed baby crying under a bench (DH35).

A Ghost is merely an "imprint of a departed soul" according to Severus Snape, while an Inferius is a "reanimated corpse" that has no soul (HBP21).

A Dementor is a creature that seeks to suck out a person's living soul by giving them the "Dementor's Kiss," a horrible punishment suffered by Barty Crouch Jr. when he was caught masquerading as Mad-Eye Moody (GF35). The first two times a Dementor attacked him, Harry heard his mother screaming as she died (PA5, PA20). The third time, when both Harry and Dudley were attacked on Privet Drive, Harry was able to summon enough happy memories to keep the creature at bay and cast his Patronus to chase it away, although Dudley was in the process of being attacked. Harry told Vernon that if Dudley's soul had been sucked out by a Dementor, they would know since he would not be alive anymore (OP2) A Patronus is a "soul guardian" which can chase away a Dementor and keep the soul safe from harm (JKR).