Muggles' Guide to Harry Potter/Magic/Unforgivable Curses

Overview
The Unforgivable Curses are curses so powerful and so malign that use of them on another human being is grounds for immediate life imprisonment in Azkaban Prison.

Extended Description
The three curses that are classified as Unforgivable are the Imperius Curse, the Cruciatus Curse, and the Killing Curse.

Mad-Eye Moody introduces Harry and his class to these curses in Harry Potter and the Goblet of Fire. Ron identifies the first, the Imperius Curse, and Moody tells how that one had caused a lot of trouble for the Ministry: often it was impossible to tell who had been acting out of their own impulses, and who had been driven by someone else. The Cruciatus curse is named by Neville, who has reason to remember it as his parents suffered greatly under it. And the third, the Killing Curse, is named by Hermione; Moody mentions that it can't be blocked, and only one person is known to have ever survived it: Harry Potter.

Harry makes use of two of the Unforgivable Curses in the books. Dueling with Bellatrix Lestrange, he attempts the Cruciatus curse, with limited results; Bellatrix says that he has to really hate someone to make the Unforgivable Curses work properly, righteous indignation isn't enough. He also uses the Cruciatus curse upon Amycus Carrow, after Carrow spits in Professor McGonagall's face, informing her that that was the reason why he did it. And when he is breaking into Gringotts, he uses the Imperius curse to control Travers, who has entered the bank with them, and Bogrod, the goblin who is ushering them down to Bellatrix' vault.

Greater Picture
It is mentioned elsewhere that the Wizarding judicial system seems oddly arbitrary, with Sirius Black being locked up in Azkaban for twelve years without trial, and the full Wizengamot, with Cornelius Fudge acting as both prosecutor and head judge, called into session to judge a case of Underage Sorcery. The fact that use of these spells against human beings is grounds for such severe punishment simply increases the apparent arbitrariness of the legal system.

Given the harshness of the penalty, one wonders then why, in the later books in the series, there seem to be Cruciatus and Killing curses flying around so frequently, and so many wizards affected by the Imperius curse. We have to assume that the author is making a point, that the laws really only affect the behaviour of the law-abiding citizens, and that increasing the penalty does not seem to have much affect on those who would break the law anyway. It also, perhaps, is meant to expose the folly of statutory minimum sentences, as we suspect that Harry had no alternative except to use the Imperius curse to enter Gringotts. Given that the curse should trigger an automatic life sentence in Azkaban, and no mention is made of allowance for extenuating circumstances, Harry should have been sent directly to prison, and definitely would not have gone on to have a career as an Auror.