Dementors are the non-human guards of the wizard prison Azkaban; they're tall, hooded figures with rotting flesh. They feed on happiness and souls and guard prisoners by sapping them of any happy thoughts, thereby compromising prisoners' abilities to even remember what life is like on the outside. Though Harry initially believes that the dementors are a fair punishment for hardened criminals, especially those in league with Voldemort, he soon learns that the dementors themselves aren't at all fair. They're indiscriminate when it comes to sucking the life out of people and will prey on an excited crowd of Quidditch spectators, innocent children, and criminals alike. With this realization, the dementors become a symbol for the corruption of the Ministry of Magic as a whole. Like the dementors, the Ministry is willing to prey on whomever is an easy target for a given crime, and it's far more interested in scaring people and acting in its own best interest than it is in actually doling out real, impartial justice.
Dementors Quotes in Harry Potter and the Prisoner of Azkaban
"The guards told Fudge that Black's been talking in his sleep for a while now. Always the same words: 'He's at Hogwarts...he's at Hogwarts.' Black is deranged, Molly, and he wants Harry's dead. If you ask me, he thinks murdering Harry will bring You-Know-Who back to power. Black lost everything the night Harry stopped You-Know-Who, and he's had twelve years alone in Azkaban to brood on that."
"Didn't they want to help, sir?" said Percy.
"Oh yes," said Dumbledore coldly. "But I'm afraid no dementor will cross the threshold of this castle while I am headmaster."
"But you were innocent!" said Hermione.
"Think that matters to them? They don' care. Long as they've got a couple o' hundred humans stuck there with 'em, so they can leech all the happiness out of 'em, they don' give a damn who's guilty an' who's not."
"Did anyone see you?"
"Yes, haven't you been listening? I saw me but I thought I was my dad! It's okay!"