The Imperius Curse (Imperio) is a tool of the Dark Arts, and is one of the three Unforgivable Curses. It is one of the most powerful and sinister spells known to wizardkind. When cast successfully, the curse places the victim completely under the caster's control, though a person with exceptional strength of will is capable of resisting it.
The fact that the curse can be defended against makes it unique amongst the Unforgivable Curses. It is also unique in that it is the least harmful of the curses (unless one counts the complete lack of free will) except if performed improperly.