Reavers are believed by most of the 'verse to be men that went insane at the edge of space and became savage. They stared into the void beyond and became what they saw... nothing. They gave into their primal nature and all that was civilized was discarded.
However, key members of the Alliance's Parliament know better, and the crew of the Serenity eventually discover their true origin. Most folks believe that Reavers are no longer human. It's easier to believe they have nothing in common with the rest of humanity.
Reavers are, at least genetically, human in more or less every way. They have no known physical mutations- the primary difference between normal humans of the 'verse and Reavers is their utter and complete madness. Showing no care for life, they are driven by an insane, homicidal aggression that manifests in the cannibalism, rape, and torture that they visit upon any unfortunate enough to come across them.
Reavers are impossible to mistake for anything else. As a symptom of their insanity, they deliberately gash and cut their own flesh, and a Reaver is often covered in open or partially healed wounds. Barbaric piercings and random bits of metal shoved into their flesh are also common, giving them a ghastly and corpse-like air. They seem no physically stronger or tougher than normal humans- merely innured to pain to the point where they care nothing for their injuries or indeed their own lives, and more willing to push their bodies to the extreme because of their madness.
Exactly how sentient individual Reavers are remains an open question. Although they clearly must have some retention of any original knowledge prior to their madness (as evidenced by their command of starships and weapons) Reavers are as without fear as they are without mercy, and they have never been seen to speak or indeed communicate (although that may be because any time they are observed they are on the hunt). The sole sound they make is an insane, wordless scream, which adds to the mystery of how a group of creatures little more than beasts could run a starship to the degree that they do. Indeed, Reaver ships may, although rarely, attack each other.
Reaver territory is centered about the rim of known space, and at least one massive gathering was present around the planet Miranda. They do not, however, live on any planet. Instead, Reavers spend their entire lives aboard their grotesque spacecraft, floating through the void and setting planetfall only to raid and pillage before leaving for the hunt once more. Normally they stay within their own territory and only venture out on raids, rather than moving en masse to attack the Alliance or the border worlds.
Reavers seem to favor close combat, although they are fully capable of using firearms. It is likely that their reduced mental abilities makes firearms a poor choice. Instead, they rush forward, occasionally firing at victims if stymied, and overwhelm their enemies in a deadly charge with cruel blades and melee weapons and their bare hands and teeth. Although they occasionally carry conventional firearms, Reavers also seem to use poisoned needles, and very frequently a form of large speargun. The aim of both weapons is probably to ensure that the victim is alive when taken. Utterly fearless, they will charge until they are wiped out - casualties don't seem to matter much to a Reaver band.
Reavers seem to prefer to subdue their victims. Those unfortunates who are rendered helpless or fall are either raped to death, eaten alive, flayed, or commonly all three, as well as other horrific depravities. After they finally escape into death, the Reavers will sew their skins into their clothing, and may very well either hang up the corpses as a gruesome trophy or strap them to their starcraft. Reaver craft are almost inevitably festooned with at least a few skeletons.
On a planetary raid, Reavers target settlements without an abundance of defense. After raping, murdering, eating, and sewing Reavers burn the settlement to the ground. Most of the time Reavers stick to their hunting territories; however, over the past few years the Reavers have been expanding from their territories. This is likely caused by the depletion of food sources as settlers steer clear of their territories.
Reaver starships are abominations: older-line models for the most part, often no longer used by other inhabitants of the 'verse. They are festooned with bits of random debris and ghoulish junk, a characteristic paint scheme of red slashes, and the skeletons of their prey, and are frequently armed with a Magnetic Grappler with which to seize victim ships. Reaver craft have a savage and funereal air, and are not in particularly good shape - they are almost inevitably running without reactor containment, which means the inhabitants are constantly being bombarded with radiation at suicidal levels. Reaver ships seem to broadcast a nightmarish chorus of screaming across all channels, although whether this is a recording used to induce fear, the howling of their inhabitants or the sounds of their victims is uncertain.
In space raids, Reavers target poorly defended ships and cargo transport vessels. They do this by catching the vessel with a Magnetic Grappler. Once on board Reavers quickly subdue the passengers and commence with their depraved pleasures. Once that's done they string up the bodies in a place where they can be found by rescue parties -- they sometimes leave a trap attached to the hull of the derelict to cripple rescue ships. This means, of course, that the ship will be left stranded- and easy prey for the Reavers if they choose to return. Once a civilian ship is boarded by Reavers, it is doomed, because the close confines of a ship are natural settings for savage melees.
Reavers in space battles display the same degree of suicidal insanity that they do on land. They seem to display the same preference for close combat when in their craft - Reavers slam into enemy starships without pause, heedless to the damage to their own hulls, and their ships may very well be armed with folding 'arms' ending in large blades that presumably may be used to rend and tear rammed victims. The sheer weight and inertia of their ships may buckle warship hulls. Their ships are also armed with conventional starcraft weapons, as well as electromagnetic pulse beams, which aid in their capturing of potential victims by shutting down electrical systems.
(Warning: This section contains a major plot point)
Reavers originated from one of the outermost planets in the solar system named Miranda. The Alliance had been testing behavioral modification on this planet through a chemical known as "Pax." The chemical was designed to inhibit aggression and anger, and was administered through the air recycling system on the planet. A tenth of a percent of the population had the opposite intended reaction, with their aggression and primal instincts boosted beyond simple madness and into the realm of animalism, robbing them of their humanity.
Reaverdom does not seem to be exclusively chemical in origin. Some victims of Reaver attacks are left alive and forced by Reavers to view the tortures they inflict on others. When this happens, the person's mind is so damaged by what they were shown, that the only way to cope with it is to begin to act as a Reaver does, eventually becoming one themselves. This was witnessed on one occassion by the crew of the Serenity when they found one such victim on a derelict ship (Bushwhacked). (Mal's knowledge of this indicates that he either has heard of such things before or witnessed it himself.)