In the Harry Potter novels, an Inferius is an animated corpse, much like a zombie. Called Inferi in plural form, they are unable to think for themselves, existing solely to complete an assigned task. The ability to create an Inferius is classified as Dark Magic, because they are usually used to cause mischief or attack people. The word Inferius appears to be related to the Latin for “below.” J.K. Rowling has not shown readers the creation process for an Inferius, but suggests that it is quite difficult magic.
Inferi first appear, briefly mentioned, in Harry Potter and the Order of the Phoenix. In Harry Potter and the Half Blood Prince, the term is defined, and Harry Potter encounters them during the climax of the book, when he and Dumbledore attempt to recover a Horcrux. According to a discussion which takes place in Sevurus Snape's Defense Against the Dark Arts class, an Inferius is an animated corpse which obeys the Dark Wizard that created it. Inferi are differentiated from ghosts who can think for themselves, but because of their non-physical existence are unable to physically act on the world around them.
Because they are used for evil purposes, Inferi are regarded as evil, and usually are thought of as enchanted objects rather than people. The source of bodies for Inferi is not made clear to the reader, although given the large armies of them which Voldemort has raised, muggle graveyards are probably a good bet. The bodies of wizards may be used to create an Inferius, but it seems likely that wizards take steps to prevent this, such as building specialized tombs. Little is known about the burial habits of the wizarding world beyond the funeral held for Albus Dumbledore, which does not appear to be typical of the wizarding world.
Although Inferi are created with the use of magic, they cannot be harmed by magic. In Harry Potter and the Half Blood Prince, Harry attempts numerous spells to fight off the Inferi attacking him and Dumbledore. Although he causes physical damage, this does not deter the Inferi. The only way to combat Inferi appears to be the use of fire. Whether fire damages the body beyond usability or instills fear in the heart of an Inferius is unknown. The use of fire to deter an Inferius suggests a metaphor of light overcoming darkness, a large theme of the Harry Potter books.