The Shadow is a warped reflection of the physical world (or Material Realm) in which everything is alive; anything of import that happens in the Material Realm may birth a reflection in the Shadow, and that reflection may live on long after the original is gone. Thus demolished buildings, ancient trees and extinct animals may all still exist there in spiritual form, as may ideas and concepts which pre-date anything in the modern world (among which are the idigam). The only exception are human beings (and similar creatures like vampires, werewolves and Prometheans): people are eerily absent from the Shadow. This may be because only these creatures possess souls (or, in the case of Prometheans, an equivalent). (Ghosts, the echoes sometimes left behind when humans die, are composed of ephemera but exist in Twilight, not in the Shadow.)
Travel between the Material and Shadow Realms is impossible because of a barrier known as the Gauntlet, except at places of great spiritual energy known as loci. Because of the Gauntlet, few mortals are aware of the Shadow; only spiritual creatures, Uratha, mages adept in the Spirit Arcanum and Prometheans of the Ulgan Lineage commonly interact with it and its denizens.
According to the lore of the Uratha, the two realms once intermingled much more freely. It was Father Wolf's task to patrol the border between the realms, keeping a balance in the affairs of mortals and spirits. When Father Wolf was slain, the Gauntlet rose between the realms. The Forsaken Uratha of the modern world have now taken on Father Wolf's role, ensuring that the denizens of the Shadow Realm do not meddle too much in the Material Realm.