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.
JK Rowling's play on words is, I think, very revealing. Dementia sucks the psyche of it's victims. Dementia patients have their lives sucked out of them by the disease.