There are three Unforgivable Curses. The use of any of them on a fellow human earns the cater a life sentence in Azkaban. Technically, আপনি shouldn't see what these curses look like until your sixth year.
The Cruciatus Curse is used for torture. Bellatrix Lestrange used it on Frank and Alice Longbottom, torturing them into madness. It produces a feeling of pain, unbearable pain. Victims will usually be writhing on the ground.
The Imperius Curse can be used to force someone to do your will. The caster has complete control over the victim. Most commonly, the victim, when the curse is removed, will have no memory of the event. Many Death Eaters, after Voldemort's first fall, claimed to be অভিনয় under the Imperius Curse. However, there is no way to sort out the liars.
The Killing Curse is used for murder. This spell is characterized দ্বারা a flash of green light and a whooshing sound. There is no way to block it. There is no countercurse. Only one man is known to have survived it: Harry Potter.