I have no idea if this what Bulletins are supposed to be used for but fuck it.

Terra's cosmology is pretty simple. There are five primary planes of existence. They are often viewed in-universe as being effectively three planes stacked on top of each other, but the truth is each is fundamentally separate. They cannot be directionally related in any shape way or form. Heaven for instance is not "above" Terra, nor to its side, or beneath it. Such a description is fundamentally nonsensical. Each of these planes are connected to other by the cosmic principle of Dervadatta ("wow, what real-life mythos did you pull that from?" I didn't, I just made up some cool-sounding shit I'll probably replace later). Don't bother trying to understand Dervadatta, 99.99% of the beings in Terra have no clue what the hell it is either and the 0.01% that do don't understand it either. 

Terra is defined by set of Akashic Principles. These are basically fundamental facts of existence. Their names are etched into Existence--if you dig deep enough, you just fundamentally know them. No one knows who named them, but most savants assume they are the names of whatever beings created the universe, with each Principle being named after the being that made them. These names are  They are:

Ishara: "Existence exists." (Obvious)

Madraga: "Existence consists of separate miniature existences inside of itself." (Existence consists of things, each of which are different from each other and able to be defined, yet as complex and grand as Existence itself)

Chroneos: "Existence is not a singular occurrence, but a progression of happenings which all facets of Being are inexorably subject to: Time."

Tanuvaya: "As time passes, facets of Existence conflict with each other." (this is far more extensive than what "conflict" probably means to you: this defines the simple fact that thing in Terra are capable of interacting with each other in any shape way or form)

Saroja: "Existence is self-sustaining. It exists because it Exists." (The universe isn't going to end for no reason--it needs no "fuel", no external source of power--it is a cyclical thing that exists solely because it already exists, as mind-screwy as that sounds)

Higreos: "Existence has always existed, and always shall exist." (whoever these "Makers" were, they created the universe outside of the concept of time--from the perspective of the universe, it is infinite and shall be infinite. The "Makers" had no need to weave themselves through the concept of time--they existed and exist outside of it, creating a weird-ass Homestuck-like situation where if they still exist they can see literally the entire universe at any time instantly, though there's a bit of a catch...)

Veritas: "No universal truth exists--there is only what those with Will inside of it make of it" (the universe does not have a particular morality engraved into it)

Akasha: "Will is the ultimate force in Existence, and all bends to it" (this rule was broken, or perhaps more exploited, with the invention of the Akashic Engine)

Terra is a planet. It forms the Terran plane, which is basically just "reality" for normal humans, animals, and the like. It is roughly similar to Earth. 

Heaven is the abode of the Devas. It is a separate space. It is simply their abode--it has no cosmological importance beyond that. Seemingly perfect in every way, it is in reality a cut-throat land of corrupt bureaucracy for those Devas who have enough power to enforce their wills and not enough to make others submit without question, and the same kind of careless hegemony for those Devas who aren't so lucky. The Terrestrial Gods may view their Celestial cousins with either envy or hate, but those in the core of Heaven have more in common with them than they think. Inside Heaven exists the Akashic Engine, a grand machine that defines Reality and forces it to follow pre-set laws (it is effectively the concept of physics reified into an actual thing).

The World of Spirits is where the dregs of the world live. It is a land of pure Will and chaos, devoid of any definable shape or form. Spirits are the formation of human fears and dreams, each one unique. Some are the basic fears of the average man and woman--others are grand narratives that gnaw at every human's mind, grand Spirit-Kings, clothed in the fears of entire cultures and living in palaces of dreams. The Akashic Engine pulls from this world to form souls for humans.

The Netherworld is the repository for every soul that died--when a human dies, their soul drifts to this world, where it lays in rest. A hundred billion glinting souls lie in these somber fields, which are infinite in size despite existing in a single glass ball in the most ornate mausoleum in existence. There is little here for humans, gods, or Asura--but there has to be a reason the Akashic Engine maintains this massive record of every soul that has ever existed...

Makai is not a place so much as a part of the World of Spirits. When the Asura turned tail and ran, they invaded that realm and colonized a portion of it--the Wasting Onyx Sun threw the Rotstar in the air, and burnt his supremacy into the very fabric of the Spirit realm. There, the Asura wait. The only connection to any other realm is one the Onyx Sun maintains, and since the realm of spirits is a complete amorphous chaos, finding Makai through normal exploratory means is nigh-impossible. In their isolated piece of colonized land, the Asura have made a twisted parody of Heaven--a grand necropolis of obsidian and mercury, from where they bide their time. Some humans who are aware of Makai prefer to call it "Hell" or "Naraka". No equivalent to Christian Hell exists on Terra: the Makers were far too above conceptions like morality to encode it into reality. 

//////////

Will is a concept mentioned a lot above here. In the eyes of the universe and those who made it, Will is the ultimate force--it is the thing that defines beings that can think, feel, and act on the world such as Gods and humanity. In the eyes of the Makers, Will is the ultimate force--the world bends to one's will. This is not how Terra currently is, thanks to the Akashic Engine enforcing Reality on the world, but it is the reason magic and all supernatural forces exist on Terra: those with enough Will can flout the defined rules of the universe. Souls are nothing but pure Will--the difference between humans and gods is that humans have a corporeal body surrounding it and gods extrude a purely preternatural body from their souls. All magic in Terra is reaching into one's souls and using one's Will to force the world to change, hence why magic is damn near capable of anything (but time travel and resurrecting dead people/things). Learning magic is opening one's "third eye" so to speak to understand that your soul can effect the world around you.