Sometimes causing horrible scarring, loss of extremeties and even death.
They are hunting spiders (transient) and avoid human contact.
In short, they are badass and very good at what they do.
You do not want to get bitten by this one.
From Wikipedia:
Recluse spiders build irregular webs that frequently include a shelter consisting of disorderly threads. These spiders frequently build their webs in woodpiles and sheds, closets, garages, plenum, cellars and other places that are dry and generally undisturbed. They seem to favor cardboard when dwelling in human residences, possibly because it mimics the rotting tree bark which they inhabit naturally. They also tend to be found in shoes, inside dressers, in bed sheets of infrequently used beds, in stacks of clothes, behind baseboards, behind pictures and near furnaces. The common source of human-recluse contact is during the cleaning of these spaces, when their isolated spaces are suddenly disturbed and the spider feels threatened. Unlike most web weavers, they leave these webs at night to hunt. Males will move around more when hunting with the female spiders tending to remain nearer to their webs.
I hear tell that hospitals in the midwest are getting an increasing number of people with recluse bites. I don't know why.