The Imperius Curse is one of the three Unforgivable Curses. The use of it will get the caster sent to Azkaban. The spell can be used to control someone, when you use the incantation Imperio.
When the spell is used on someone they feel a detachment from events around them and a vague happiness.
The control is not perfect. With sufficient willpower a victim of the curse can fight the caster's commands and even throw it off entirely.
 Known Users
Barty Crouch, Sr
Barty Crouch Jr.