Inferius

An Inferius (pl: Inferi) is the dead body of a human reanimated by a Dark Wizard. They have no free will, and cannot think; their purpose is merely to serve as puppets of the Dark Wizard. They were used by Voldemort in his first uprising against the wizarding community, and it is rumored that he is again using them.

Defence against Inferi
Because Inferi are creatures of the dark, they dislike light and heat. The most effective spell against them should therefore be a fire-summoning spell, such as Incendio. Other spells might work against a few Inferi, but might not be useful against a whole army of them.

Encounters

 * In Harry Potter and the Half-Blood Prince, Harry Potter and Albus Dumbledore encountered an army of Inferi in their attempt to steal one of Voldemort's Horcruxes. Dumbledore successfully repelled them by conjuring a ring of fire big enough to surround both Harry and he.
 * Not exactly counted as a real encounter, but worth mentioning: Mundungus Fletcher was arrested and put into Azkaban because he disguised himself as an Inferius.