Reavers are not actually aliens, well no more than the humans in the series. In a television series with no aliens what isoever, the void is filled by Reavers. Reavers so many are told are men who have gone to the far reaches of space and have gone mad. Their appearance has changed from looking human to looking monster like. They rape, pilage including humans, in fact any living creature they can lay their hands on.
The truth is much more sinister than the legend. The Reavers are an experiment gone horribly wrong. The main authority in the Firefly solar system is the Alliance who control the inner worlds. They lost control of the outer worlds. They devised a chemical agent known as Pax on the planet of Miranda. The chemical was designed to make people calm and docile. For the majority it worked but some, it had horrible consequences. Some people stopped doing worked, stopped eating and gave up on everything. For others, it had the opposite effect, it turned the humans into the Reavers.
Reavers predominantly inhabit an area of space near the planet of Miranda which is where the agent was released. The Alliance told people the planet was unstable, unable to be terraformed and everyone should stay away.