The Reavers are one of the primary antagonists of the science fiction show Firefly, and of the subsequent movie Serenity. The Reavers actually only appear in two of the 14 episodes of the television show, but play a more important role in the movie.
To call the Reavers "antagonists" is somewhat of an understatement. The Reavers are pure aggression and violence, self-mutilated and capable of every type of torture and cannibalism imaginable, and presumably many that are unimaginable.
In the pilot "Serenity" (not to be confused with the motion picture of the same name), the Reavers are first mentioned, and it is said that they went insane from being exposed to the darkness of space. They are never shown directly, although their ships, and the results of their raids, are shown. What role the Reavers would have played had the original series been allowed to develop is unknown. It could be that the origin story told in the movie was planned from the start, or it could be that they would play a different role in any arc that was planned for the show.
In the movie "Serenity", it is shown that the Reavers were the accidental product of an experiment in social engineering. The planet Miranda, when being terraformed, had a drug introduced into the air that would dampen people's aggression. For most of the people on the planet, it did just that, dampening their aggression until they became totally apathetic and died. However, for a small number of the subjects, the drug had the opposite effect and drove them to madness, and these were the first Reavers.
The Reavers come across as very frightening, although most of the fright comes from people's reaction to them. Like many frightful things, the terror they cause is worse than seeing them directly. I was actually somewhat disappointed during the first full appearance of Reavers during the climactic scene of "Serenity", because they appeared to be fairly standard "zombies" of a kind that has been done before in many movies.
"Serenity" does put the Reavers into a more clear thematic perspective. Much of the television show "Firefly" seemed to mostly be a story of the conflict between the structured civilization of the Alliance, and the much more wild life of the criminal protagonists. "Serenity" then expands that continuum, showing the protagonists of the ship as lying between the authoritarian, civilized alliance and the insane aggression and violence of the Reavers.
There are two large issues I have with the Reavers, one internal to the story, the other external. The Reavers live in a large colony, and operate technologically advanced space ships, and seem to have enough cohesion to make plans, move in groups and coordinate their actions. Given their penchant for overwhelming, unstoppable violence, it seems unlikely that they would be able to continue as a society, even for the purposes of war making. The other question is one that is rather tenuous, but unavoidable: given the fact that Firefly/Serenity is a "space western", the presence of bestial humans who are merciless and can therefore be killed with impunity, has some obviously unsettling implications.