The Cruciatus Curse (also known as the Torture Curse) is one of the three Unforgivable Curses of the wizarding world. It is a curse of torture, inflicting excruciating pain on a victim. This makes it popular among the Death Eaters for use on both fellow wizards and Muggles. Considering the fact that this curse doesn't physically harm the victim, it is possible that it only stimulates pain receptors. Whenever a person explains he/she wants to use it, they say they will Cruciate.
To successfully perform this curse, the wizard or witch must possess a deep desire to cause the victim pain. The curse doesn't work if there is a lack of fundamental sadism to inflict prolonged, excruciating pain.
The pain caused by the Cruciatus Curse is described as worse than "one thousand white-hot knives, boring into the skin." This pain can cause permanent mental injury because of the prolonged trauma the curse causes. Presumably, such permanent effects occur if the victim is exposed to a particularly intense curse for a long period of time. If the curse hits an inanimate object, it may cause it to shatter instead.
Once cast, there is no spell that can defend against the Cruciatus Curse. However, there are still defensive options available against it. The spell can be dodged by hiding behind a solid object, and a particularly strong-willed wizard can merely resist the pain until the spell is lifted (though, prolonged expose can still cause permanent mental damage). Another option is to interrupt the caster before they can finish speaking the incantation.
The Cruciatus Curse was invented during the early middle ages, by dark witches or wizards. The curse was created for torture purposes, but has also been used effectively in duels. The curse was at some point once very popular.
The Cruciatus curse is one of the three Unforgivable Curses, which means it is illegal to perform on a fellow human being, unless one can prove they are under the effects of the Imperius Curse when performing it.