Outside is... well, no one is really quite sure. No mortal truly knows the nature of Outside, though many will claim otherwise. To most Christians, Outside is God's Heaven and a fiery Hell of brimstone and sulfur, while Buddhists believe Outside contains the Naraka (and possibly Nirvana as well). To the Dwarves of the Hindu Kush, Outside is a realm of eternal sandstorms, an endless exposed wasteland scoured by lightning and dust. Regardless of the specific details, "Outside" is the catch-all term used by theologians, philosophers, mages, and others for realms beyond that which can be observed, pieces of reality that sometimes bleed into our own in strange ways.
While the world certainly has its fair share of agnostics and atheists, very few deny the very existence of Outside. One of the major reasons for this is the occasional appearance of supernatural entities which seem to have no place in this world - demons, angels, asuras, and other beings, often with close ties to belief and mythology. While the world certainly contains many magical creatures, Earth's native fauna all seem to fit into an ecology of sorts, no matter how bizarre, while things from Outside always seem slightly out of place. It is known that some such creatures can be summoned intentionally by powerful magic, while others seem to cross the boundaries between worlds of their own volition.
Given its reliance on something external to the practitioner for power, the magic of shamans is heavily dependent on entities from Outside. In fact, some argue that all shaman magic comes from such creatures, but the precise nature of those spirits of stone and tree that many nature-oriented shamans bargain with may be impossible to pin down. That said, whatever deities there may be, or whatever spirits are perhaps imitating them, are generally agreed to reside Outside, as well as the demons and nightmares to which so many cultists gleefully sacrifice.