The unforgivable Curses are three of the most powerful and sinister spells known to the wizarding world, and are known tools of the Dark Arts. They were first classified as "Unforgivable" in 1717.

They are the Killing Curse (Avada Kedavra),

the Cruciatus Curse (Crucio),

and the Imperius Curse (Imperio).

Using any of these three curses on another human being, Muggle or wizard, will result in a life sentence to Azkaban, unless of course, there is evidence that the caster did so under the influence of the Imperius Curse. In the first wizarding war of 1997 to 1998, the Aurors were permitted to use them while under he who must not be named's rule. But these curses were then made illegal again, following he who must not be named demise.

The Unforgivable Curses are very powerful, and their use requires both great willpower and great skill in order to bring about the effects. 

Avada Kedavra.

The incantation of Avada Kedavra causes a blindingly intense green bolt of light to shoot from the end of the caster's wand, which, on contact with the victim, results in instant death. The Killing Curse has no counter-curse or blocking spell, although the green bolt may be dodged or blocked with solid objects. This curse was known to be used by he who must not be named and his fellow death eaters, the only person to be known to ever survive the killing cure was Harry potter, also known as the boy who lived. This curse has been known to have taken many people and creatures lives.

The Cruciatus Curse.

The Cruciatus Curse Inflicts excruciating pain on it'd victim, it requires the verbal incantation and the use of a wand. The effects of the curse depend upon the desires and emotions of the caster, to produce the excruciating pain implied by the name, one must, desire to cause pain purely for their own sake. The extreme pain inflicted by this curse makes it uniquely suited as a form of torture, and was used regularly by the he who must not be named death eaters. The cruciatus curse as a red flash of light when casted. 

The Imperius Curse.

The Imperius curse causes the victim to perform the unquestioned bidding of the caster, it places the victim in a trance or dreamlike state, leaving him or her under the complete control of the person who uses the spell. The experience is described as a "wonderful release" from any sense of responsibility, the victim is often calm and ok with doing things they would be physically incapable of normally doing, to committing horrible crimes they never would. They will obey any order, This curse can also works on animals. This curse is also one of the favourites of the Death eaters, some even tried to get out of punishments by saying that they were under control of this said curse. Resisting the Imperius Curse is possible but extremely difficult, only those of a strong will can achieve it. When casted, this curse has a rainbow effect at person's Head. 

And there we have the unforgivable curses, use of any of these deadly curses will get you landed in Azkaben for life.