There are three unforgivable curses within the Harry Potter books. These curses are illegal under every circumstance and will result with the person who casts them being put into Azkaban.
The first would be the Imperius curse. This curse causes the person you cast it onto to do whatever you want them to. If you have the mental power, you can actually fight this curse. Harry was the only one out of his class to show any real power over this curse. While in class with the fake Mad-Eye Moody during The Goblet of Fire, Harry was able to fight against the curse and actually break free from it.
The second curse is the Cruciatus curse. This is the torture curse. Frank and Alice Longbottom lost their sanity to this curse after it was put on them by Bellatrix (an act which she was imprisoned in Azkaban for). Bellatrix also used this on Hermione, and Harry on one of the Carrows.
The last curse is Avada Kedavra – the killing curse. This one is simple if you are hit with this curse you die.
For any of these curses to work, you really need to mean them. In The Order of the Phoenix, Harry tries to use the Cruciatus curse on Bellatrix. She is merely stunned and knocked to the ground. He didn’t have the true feelings behind the curse in order for it to be executed properly. However, when, in The Deathly Hallows, Professor McGonagall is threatened by a death eater, Harry is able to perform this curse perfectly. I think both were stunned at how well it worked, but as Harry simply puts it, “he shouldn’t have spat at you”.