The Imperius Curse used by pointing at a person and saying the incantation Imperio is an Unforgivable Curse. The use of it will give you a one way ticket to Azkaban where you will stay for a period of 12 years. The curse, which has no origin, enables the person who had cast it to control the target like a mind puppet. They can use them to kill people, steal or go under cover in Ministry buildings and get secret information. It is very hard to tell whether a person is under the Imperious Curse or not, but if they start to act unusual a call to the Ministry of Magic is suggested. Harry described it, when being cast, as a "strange feeling going down your arm".
This curse may get its incarnation from the Latin word imperiosus, meaning "commanding, powerful and mighty".
Imperio is a common spell in the later books, being used by Barty Crouch Jr (polyjuiced as Mad-eye moody) on a spider to show the class as the first. This is one of the unforgivable curses Harry Potter has used (Crucio as the other).