Muggles' Guide to Harry Potter/Magic/Inferius

Overview
Inferi are corpses that seem alive.

Extended Description
Inferi are actually animated by the will of a wizard. They are greatly feared because, being already dead, they cannot be killed, and are effectively fearless. Animation of Inferi is a very dark form of magic, and Voldemort made great use of them in his attempts to take over Wizarding society before Harry was born.

It appears that Voldemort has used Inferi to protect one of his Horcruxes; we see them in Harry Potter and the Half-Blood Prince. At this point, we learn also that they can be stopped by fire.

We learn in that book, also, that Mundungus Fletcher has been imprisoned in Azkaban for impersonating an Inferius. It is in relation to Mundungus that Inferi are described; they are mentioned beforehand, but it is only when a student brings up the story of Mundungus' imprisonment that the Defence Against the Dark Arts teacher, Professor Snape, actually describes what they are.

Analysis
In the cave of the Horcrux, there are a large number of Inferi lying in wait under the surface of the water. We have reason to believe that those Inferi have been there for at least fifteen years, most likely much longer, as Kreacher tells us that they were there when Voldemort was initially hiding that Horcrux, before Harry was born. Thus, we must assume that either some part of the spell used to create Inferi, or some additional spell that is not discussed, must halt the natural processes that would result in the dead body decomposing.

Greater Picture
In Harry Potter and the Deathly Hallows, we meet the animated corpse of Bathilda Bagshot. While her body does contain Voldemort's pet snake Nagini, it is uncertain whether Nagini is controlling Bathilda, or whether what remains of her has been made into an Inferius. It is true that her remains have not deteriorated significantly in the several months since her passing, so some of the magic that preserves the Inferius must have been used on her.