"When they get near me--I can hear Voldemort murdering my mum." -- Harry Potter
Horrible, spectral creatures, hooded and robed, which feed on human emotions. Dementors drain ‘peace, hope, and happiness out of the air around them,’ according to Lupin (PA10). Even before a Dementor is seen, its presence is obvious; they are surrounded by an unnatural darkness and terrible icy cold (PA5). Dementors affect even Muggles, although the Muggles can’t see the foul, black creatures. They were the guards at Azkaban and made that place horrible indeed. The Ministry used Dementors as guards in its courtrooms as well (GF30, DH13).
When they breed, they create chill mist which permeates everything (HBP1). They drain a wizard of his powers if left with them too long. A Dementor's breath sounds rattling and like it's trying to suck more than air out of a room. Its hands are "glistening, grayish, slimy-looking, and scabbed". It seems to exude cold (PA5). A Dementor's last and worst weapon is called the Dementor's Kiss. The Dementor puts back its hood and clamps its jaws on the mouth of the victim and sucks out his soul, leaving him an empty shell, alive but completely, irretrievably "gone." (PA12, PA20) (from The Lexicon page 82).
There are certain defenses one can use against Dementors, specifically the Patronus Charm.