Reavers are insane, cannibalistic humans from the Fireflyverse. Various theories circulate around the planets as to their creation. The most popular theory appears to be that they are 'just men' who looked upon the uninhabited space beyond the edge of the system and 'went bibbledy over it'. However, it is revealed during the movie Serenity that they were originally created by the Alliance pumping a chemical known as G-23 Paxilon Hydrochlorate, or Pax, into the atmosphere of the planet Miranda.
Agents encountering Reavers on a mission are advised to portal away with alacrity, and to never under any circumstances allow them entry to HQ. Reavers are deadly and apparently feel no fear or pain. Engaging them in combat is not advised, though if it is unavoidable, they can still be killed with standard firearms and melee weapons.
Of note, the Reavers are one of a very short list of Sue assassination methods that the PPC has strictly forbidden the use of. This is probably because of what they do to their victims, described thus in the canon: "They'll rape us to death, eat our flesh, and sew our skins into their clothing. And if we're very very lucky, they'll do it in that order." Even for the PPC, this is pushing things, and probably counts as torture, though on one occasion Flip Finley and Rez Montrose executed a River-replacement Sue in such a manner; it is possible this is what caused the rule to be put in effect.