Earth is a realm of mortal beings called "Earthly Ones." Although much of the enviroment is subject to the laws of nature and physics, balances between the forces Heaven (Paradise) and Hell can have a major impact on the Earth.
Travel between Earth and Limbo - an unearthly realm between Paradise and Hell - can be done freely by Angels and Devils (and such access is generally restricted), or Earthly Ones, if they possess the right knowledge or magical access. Travel between Earth and upper or lower realms (Paradise and Hell) is done through special locations, like Golden School, so the High and Low Spheres can better monitor movement and activity between the earthly and unearthly realms.
Angels and Devils use mortal people in a long-held conflict to guide Earthly Ones to moral greatness or complete villainy, with strict rules - called the V.E.T.O. - to allow for the Earthly Ones to maintain their freewill.