No one knows exactly how Demon Realm came to exist, but it remains exactly parallel to our dimension. Thus it is thought that it came to be through the same set of events our dimension was created through. The dimensional wall weakens in some areas, and the wall can be temporarily removed all together in some cases.
As its name suggests, Demon Realm is populated by a number of demons who come in many shapes and forms, both sentient and unintelligent. More suprising however is the minority human population. There are a few handfuls of half-demons, but these are generally looked down upon by
Being parallel to our universe, it is obvious that Demon Realm is a term applied to the area itself by its denizens. In all reality, it should be a planet as well.