Forget everything in the White Wolf books. We are going to use a similar scheme, but one a little more accurately reflecting the beliefs of the majority of religions and a little less artificial and tied to the metaphysical trinity. It isn't hard to grasp and is actually a bit more intuitive for most folks, since it is tied closer to what most people think of as spiritual realms.
The Middle World is the Earth and its spiritual aspect, what the White Wolf scheme calls the Penumbra, except that there are no seperate Penumbras for the three different Umbras, there is only one. Ghosts, nature spirits, and demons all share the same spiritual space in the Middle Realm. The barrier between material reality and spiritual reality in the Middle Realm is called the Gauntlet, the Shroud, the Shield of Scathach, and a number of other names, depending on who you ask, but they all amount to the same thing.
The Upper World is called Asgard, Heaven, the Celestial Courts, or the Aetherial Plane. It is the conceptual realm, shaped by the dreams, concepts, and beliefs of humanity. It contains paradises, mythic lands, and even embodied concepts. The beings which dwell here are alternatively called gods, angels, hsien, tuatha de daanan, or archetypes in the collective unconscious. The beings here represent art, culture, permanence, and ideologies. Travel to this realm is achieved by ascent from the spiritual reflection of the Middle Realm and often involves flying, climbing a World Tree or Mountain, or travelling upstream on a spiritual river.
The Lower World is called the Yomi World, Hell, the Infernal Reaches, or the Underworld. It is the emotional realm, shaped by passions, instincts, and the dark forces of reality like death and destruction. It contains hells, lost or forgotten lands, and the smoldering remains of cities and whole civilizations. The beings which dwell here are called demons, ghosts, baka, fomorians, or shadows. These entities represent death, decay, corruption, and destruction. Travel to this realm usually involves going underground or into a large body of water, descending the World Tree, or travelling downstream on a spiritual river.
Most spirits have an affinity for one of the three levels of the spirit world. Those with an affinity for the Middle Realm are usually nature spirits, animal totems, or ghosts with unfinished business. It is also possible for beings to shift in their affinity. For example, gods who are killed, defeated, or just forgotten tend to migrate from the Higher Realm to the Lower and the occasional hero, after death, has ascended to the Higher Realm to join the gods rather than descend into the Underworld.
Lastly, it should be noted that Mages tend to perceive spirits according to their paradigm, so that a trickster god from the Upper Reaches will appear to an African witch doctor as Eshu, to a Native American shaman as Coyote, and to a Norse witch as Loki. Also, spirits are limited in their ability to interact with the Middle Realm since the construction of the Gauntlet and yet they rely on humanity to a large extent for their existence (or their misery) and so many are receptive to any Mage's offer of a chance to get a foothold on Earth in exchange for some favor. Of course, some spirits get around this through use of possession or other means to breach the Gauntlet.
A Node, Regio or Shallowing is merely a place where the Gauntlet is thin and a Mage can contact the spiritual world easier. Since quintessential energy flows more freely in the spirit worlds, as it isn't locked into patterns, these are also good places to recharge such energy. While most border on the spiritual side of the Middle World, some border on the other realms. Nodes bordering on the Lower World are called Haunts and are generally associated with forces typified in this realm or merely with the beings which dwell there, like wraiths and demons. Also, nodes that are underwater or underground often border on the Underworld. Similarly, some Nodes are attuned to the Upper World and are usually associated with the forces of that world and its inhabitants. Churches and temples to gods are the most common ones, but also nodes on mountain tops often border the Aetherial Reaches.
It probably isn't surprising that Horizon Realms and the like aren't really going to be a part of this scheme, which we feel isn't a bad thing. The game is about humanity and should, thus, focus on earth. This scheme here is decidely humanocentric and is meant to be. Anyway, places of power are usually on nodes and that is as close as they get to the spirit worlds. Also, as the far reaches of space and the depths of the ocean are explored by the Order of Reason, the Upper and Lower Worlds are beginning to get the same treatment that the Middle World got. That is, there is now a Gauntlet in parts of both these worlds that seperate the spirit realm from a cold, dead material realm. However, this is barely undertaken and most of these places still have no Gauntlet, so that your Void Seeker who travels too far into the depths of the ocean is going to encounter the monsters always rumored to be there, who are nothing but demons and fomorians.