Soul

Sirius Black's Soul.

The soul is the immaterial part of a being which is held to live on after death in many belief systems. Wizards and witches, like Muggles, have sought to uncover the nature of the soul, particularly its role in magic.

While the human body relies on the soul to have their own unique mindsets, a torn soul that is encased in a Horcrux is dependent on the object's well being. That is to say, if the human dies, the soul would be able to move on beyond the Veil or return as a ghost, while if a Horcrux is destroyed, the soul within would die away and disappear.

Damage to souls

Ripping

Voldemort's mangled soul.

Dumbledore: " [Draco's] soul is not yet so damaged. I would not have it ripped apart on my account."

Snape: "And my soul, Dumbledore? Mine?"

Dumbledore: "You alone know whether it will harm your soul to help an old man avoid pain and humiliation."

— Albus Dumbledore and Severus Snape discuss Dumbledore's final wishes

Murder is such a serious crime that it causes the soul of anyone who commits it to be torn apart. This damage can be undone if one feels genuine remorse, but the process is extremely painful, and can even be deadly. It is also possible that one's motive for ending the life of another person might influence whether or not their soul is damaged.

Partial removal

"Well, you must understand that the soul is supposed to remain intact and whole. Splitting it is an act of violation, it is against nature."

— Horace Slughorn reluctantly explains Horcruxes to young Tom Riddle

Wizards and witches who have committed murder can use a spell to place a torn fragment of their soul inside an external object, called a Horcrux, which anchors their soul to the living world, rendering them immortal. Creating a Horcrux makes the part of the soul left inside one's body very unstable, and, for obvious reasons, is widely considered the most wicked of all the Dark Arts.

If the soul fragment has been detached from the body for a long time, the destruction of a Horcrux containing such a fragment might not be felt by the owner.

A soul that has been torn apart and partially removed from the body will leave the remaining portion of the soul weakened, and that if they remain as such, they would be trapped in Limbo when they die, unable to move on or return as a ghost.

Complete removal

"You can exist without your soul, you know, as long as your brain and heart are still working. But you'll have no sense of self any more, no memory, no...anything. There's no chance at all of recovery. You just - exist. As an empty shell. And your soul is gone forever...lost."

— Remus Lupin explains the effect of the Dementor's Kiss to Harry Potter

It is possible for a person to live without any soul at all so long as their heart and brain are still functioning. However, without a soul, a person is left in an incurable vegetative state — they have no awareness of themselves or the world around them. The soul cannot be recovered once it has been lost.

One known method of removing the soul from the body intact is the Dementor's Kiss. The Dementor — a creature which is itself soulless— locks its mouth over that of its victim and sucks out his or her soul. The Ministry of Magic has in the past used the Dementor's Kiss as a form of punishment.

Ghosts

Nearly Headless Nick: "Wizards can leave an imprint of themselves upon the earth, to walk palely where their living selves once trod. But very few wizards choose that path."

Harry: "Why not? Anyway - it doesn't matter - Sirius won't care if it's unusual, he'll come back, I know he will!"

Nearly Headless Nick: "He will not come back. He will have...gone on."

— Nearly Headless Nick and Harry Potter talk about the afterlife

A ghost is the imprint of the soul of a witch or wizard who chose to linger in the world of the living after death. According to Nearly Headless Nick, few people actually choose this fate, because it means they will never move beyond the Veil, as most do.