The dementors are "soulless creatures... among the foulest beings on Earth": a phantom species who, as their name suggests, gradually deprive human minds of intelligence. They are the guards of the wizard prison, Azkaban, until after the return of antagonist Lord Voldemort.
In the books, dementors have a generally human shape, approximately 3 metres (10 feet) in height, covered in dark, hooded cloaks that reveal only their decayed-looking hands. Beneath the cloak, dementors are eyeless, and the only feature of note is the perpetually-indrawn breath, by which they consume the emotions and good memories of human beings, forcing the victim to relive its worst memories alone. According to the author, dementors grow like fungi in dark, moist places, creating a dense, chilly fog. Although they are implied to be sentient, this is left ambiguous. The presence of a dementor makes the surrounding atmosphere grow cold and dark, and the effects are cumulative with the number of dementors present. The culmination of their power is the 'Dementor's Kiss', wherein the dementor latches its mouth onto a victim's lips and consumes its psyche, presumably to leave the victim in a vegetative state. Dementors are invisible to Muggles, but affect them otherwise identically.
Despite their attachment to human emotion, dementors seem to have difficulty distinguishing one human from another, as demonstrated by Barty Crouch Jr.'s escape from Azkaban, wherein they could detect no emotional difference between the younger Crouch and his mother. Their sensitivity appears less precise in proportion to the emotion's 'complexity'; this particular weakness enabled Sirius Black, an Animagus, to escape Azkaban by transforming into a dog. The principal method against them is the Patronus Charm, which both protects its user and repels the dementors. Chocolate is an effective first aid against mild cases of contact.
Protagonist Harry first encounters dementors during his third year of school, when they are sent to guard Hogwarts against Sirius Black. Being reminded by their presence of his parents' murder by Voldemort, Harry asks Remus Lupin for assistance, and thus acquires the Patronus Charm. At the end of Order of the Phoenix, the dementors of Azkaban stage a mass revolt against their employers to join Voldemort, who permits them nearly-free access to victims. In Deathly Hallows, the Ministry, under the control of Voldemort, uses dementors to punish Muggle-borns. The dementors also take Voldemort's side during the Battle of Hogwarts. After the appointment of Kingsley Shacklebolt as Minister of Magic, dementors are removed from Azkaban, and the Ministry contains them by limiting their numbers.
Rowling, by her own account, created the dementors after a time in which she, in her own words, "was clinically depressed". Dementors can therefore be viewed as a metaphor for depression.