Imperio is one of the three Unforgivable Curses. The victim is placed under the total control of the wizard who casts the spell. It is possible to break free from this control, but it is incredibly difficult, and takes real strength of character.
Professor Moody uses the Imperius Curse on a spider as a demonstration for the fourth-year students. Moody forces the spider to leap from his hand and swing backwards and forwards as though on a trapeze. The spider does a backflip, landing on the desk where it begins to cartwheel in circles, before being forced to rise onto its hind legs and tap dance. The Curse can be used for less amusing means; Moody says that he could make the spider jump out of the window, drown itself or throw itself down one of the students' throats.
There were a lot of witches and wizards controlled by the Imperius Curse when Voldemort was in power. It was a hard job for the Ministry of Magic, who had to sort out who was being forced to act and who was acting of their own free will.
Professor Moody places various fourth-years students under the influence of the Imperius Curse during their Defence Against the Dark Arts class. He uses the curse to command the students to do a number of extraordinary things, such as hopping around the room singing the national anthem, imitating a squirrel and performing astounding gymnastics.
Voldemort performs the Imperius Curse on Harry, attempting to force the boy to tell him to not perform the Cruciatus Curse on him again. Harry battles against the force of the Curse, which entices him to tell Voldemort 'no', and successfully resists it.
Katie Bell is placed under the Imperius Curse by someone in the bathroom at the Three Broomsticks pub. She is made to deliver a parcel concealing a cursed opal necklace as a surprise for someone at Hogwarts.