Regular people that were on the planet "Miranda" and were drugged by the Alliance to make them calm, but as Joss Whedon says, 'Our greatest impulses are tied to our weakest'. They had an adverse reaction and now are insane. They mess up their own faces with other peoples flesh and they will take over ships, Rape, Kill, Eat, and use your flesh for clothing. And if you're lucky, they'll do it in that order.
You may notice that on Firefly the phrase "the Reavers" was never shown written down but was only presented spoken aloud.
While the term for these zombie-like monsters is commonly spelled "Reavers" in fan literature, that spelling arose before the film Serenity revealed what they really were: people on whom an experimental pacification chemical had worked in reverse.
They were not "The Reavers" but "The Reverse" - which could not be shown in writing without giving away the secret.