Harry first encounters Inferi floating in the lake of the cave that Voldemort chose as a hiding place for Slytherin's locket. Voldemort summoned the Inferi to guard the Horcrux. Dumbledore advised Harry not to touch the water within the lake for such purposes not to awaken the Inferi. They aren't harmed by magic, but can be hurt by fire (HBP26).
It is known that Voldemort used Inferi in the First War (HBP9). Mundungus Fletcher was arrested and sent to Azkaban for impersonating an Inferius in order to carry out a burglary; Severus Snape tells this to Seamus Finnigan while teaching Defence Against the Dark Arts, saying in a bored tone, "[The] so-called Inferius was nothing but a smelly sneak-thief by the name of Mundungus Fletcher." (HBP21)