The Outer Planes

If the Inner Planes refer to the Elemental Planes, then the Outer Planes refer to... Everything else, I guess. Connected through the Astral Sea or other Transitive Planes, these planes are home to incredibly powerful creatures, gods, monsters, and otherwise... As well as not powerful things. Some version of a squirrel exists on every single one of these planes. If there are Planes free from these little bastards, I'm moving there.

The Overall Structure
The Outer Planes are a much less defined grouping of the Planes of Existence. There is no greater structure to all of the planes, though there are many connections between them, such as the Transitive Plane of the River Styx and it's tributaries. Many people, somewhat ignorantly, group many of the planes into the "Upper" and "Lower" planes, sometimes supplemented with the "Middle" planes, "Supplemental" planes, "Travelling" planes, and/or the "Gate" planes. For the sake of categorization, we'll do the same, though it's worth knowing that there is no important distinction between these, in a planar sense. They all function under the same general rules for a plane of existence on the astral sea.

Arborea
Arborea is a plane of decadence. It has many connections to both the positive energy shade(That is, the Feywild) and the quasiparaelemental Plane of Growth, and is home to many celestial creatures, Lillendi being the most common, but also many Fey creatures, and many mortal creatures. It is also home to some Fiends, Succubi being one(And, in fact, one of the most populous creatures on the Plane as a whole). Arborea is split into many internal layers, such as Arvandor, a place of forests and fields where there are many naturally-occurring portals to the plane and shade of Growth, and Olympus, the mountain and home to some gods.

Bytopia
Bytopia, the Twin Paradises, once lived upto that name. They are two infinite planes with opposing gravity, joined by a pair of mountains which meet at the summits. In the Era Before Time, these were perfect paradises which resembled Arborea and Elysium in some areas. However, near the end of that era, the giants and dragons in their endless conquests laid eyes on Bytopia, and now with one layer(Dothion) home to giants and giantkin and the other(Sherrok) home to dragons and dragonspawn, the paradises are wracked by a semi-permanent war. Not that it ruins the paradise for everybody.

Elysium
Elysium is a plane split into six infinite layers, with sub-realms locked inside the infinite cube of this plane. Elysium is a plane, home to many, many celestial creatures, though less hospitable to most other creatures. In Elysium, the Divine Purviews take much more clear form; The earth is divine and held with pseudo-godly power, easily malleable to celestials which bear the Domain of Earth. This is the nature of the entirety of the plane, even with the native creatures. Even visitors, in mere days, begin to take these traits.

Arcadia
Arcadia is the natural home of Fey. Yes, not the Feywild. Fuck you. It's the plane of Dreams, and in the Era Before Time in Nexus, the Fey of this plane claimed the shade of positive energy, what is now known as the Feywild, while the dragons, giants and the like fought over the Material plane and the mortals within. Not that their wars have ended.

Dragon Eyrie
The Dragon Eyrie is home to the Dragon Idols Pantheon, for the most part. Each Idol has a domain, at least. Asgorath, Astiballor, and Zorquan's bodies are ensconced within the Eyrie's depths, and continue to provide subtle magic and guidance for the dragons who live there. The Eyrie is an impossibly large mountain which occasionally plateaus in places to house a desert, tundra, or ocean, and is split into these physical regions to give home to the many dragons of the plane. These are not planar layers, simply impossibly large divides in the physicality of the plane, which dragons can cross via flying or whatever.

Ostoria
Ostoria, sometimes called Lost Ostoria by Giantspawn living in an area with no active connections to the giant-plane. Ostoria is home to the Giant Pantheon of Idols, headed by Annam, the All-Father. It is split into many regions, though these are not separate layers, but actual physical divides, as in the Dragon Eyrie. Because Ostoria as a plane is a mountain surrounded by desert and jungle, the divides of Ostoria are literally; The Lower Mountain, the Undermountain, the Mountain Peak, the Desert, the Forest, the Tundra, the Clouds, etc. Each region is the home of a type of Giant and their associated Giantspawn, in addition to their related Ostorians proper.

Erebus
Erebus is a place of spirit, and a place of penance. Across it's many layers, there are incredible realms whose entire purpose are the purification of the Spirit. Most mortals, being that they are not Spiritual beings, have no use for this plane, however the Lycanthropes of Azzeranni, whom are beings of half-spirit, make frequent use of this plane for a variety of reasons.

Stethelios
Stethelios, or the Plane of Time, is the plane which governs all passage of time. To be wholly disconnected from Stethelios would be to completely negate time, and while there are some planes which theoretically accomplish this, none have been officially catalogued by any reputable sources. Nearly all of the time-dilating magics in Nexus in some way effect the connection to Stethelios, allowing it to pass more quickly or more slowly, or even seem to stop entirely for a moment.

Arcadian Gateway
The Arcadian Gateway connects Arcadia to the Astral Sea; They do not connect directly anymore. This is because the Fey wanted to stay out of the many Astral conflicts, like the Blood War and the Golden Crusades, among other things. So they cut off a portion of the plane and turned it into this Gateway plane. Much harder to get through.

Delta Aurus
The Delta Aurus is the Gate Plane through which the river Styx flows into the river Oceanus. It is loosely managed by a Kirin known as the Gatekeeper, though the Gatekeeper allows all creatures through unless they have intention to damage either of the rivers, or the gate plane itself.

Delta Ferra
The Delta Ferra is the Gate Plane connecting the rivers Oceanus and Styx, specifically where the prior flows into the latter. It is managed by a coalition of Devils, which use the opportunity to prevent interference in the Blood War from the celestial host, and where they root out many Daemons which escape through the Delta Aurus.