The Tree of Life
This is what Michel Borsavin has told the Aces about how the universe works according to the Kabbalah. Some of the details may not be true in the Four Aces universe (especially about which beings live in which Sephiroths), but this is how most occultists in (the game world's) Europe and the Americas think the universe really works. But that doesn't mean it includes everything. There may well be worlds beyond the Sephiroths.
Actually, the Aces already know this because Yukio's alternative take on "Otherworlds" (other planes of existence) has held up, at least to some extent (for example, when he helped the spirits of Japanese at Ground Zero). But that only begs the question of where the boundaries between alternative worldviews lie. Wherever they are, though, it is possible for one person to comprehend and work within different worldviews. (So Yukio can learn the Kabbalah, George can learn from Yukio, and Ken can master both Western and Eastern traditions.)
(By the way, this is one of the rare cases when the Witchcraft rulebooks trump real-life sources. There are lots of different takes on the Kabbalistic universe [writers like Dion Fortune and others], but if the Aces ever explore the Sephiroths, the rulebooks are going to be the primary sources for what the experience will be like.)
The diagram below is called 'The Tree of Life'. It meshes well with the rulebooks, except for including Da'at, which they haven't addressed yet (though they've hinted at it). The major Sephiroths are described in the Mystery Codex, and the introduction to The Book of Hod gives a quick overview of all ten.
Some of the comments below came from Michel, but most of it is from George's studies in Katsoulakis' library.
Kether (KEH-ther) - Dwelling of "the Creator." No one who goes into Kether ever comes back, so there are no descriptions of what might be there. Most Kabbalists say this is where beings go to "join" with the Creator, whatever that means.
Chokmah (CHOHK-mah) - The primal universe, home of the primordial Sephyr. Kabbalists say the Sephyr were the actual architects of the universe as we know it. This is their realm, but what they are like, no one can say.
Binah (BEE-nah) - A universe of pure energy; Elemental planes and Elysium of the Seraphim. This is where elemental spirits and the angels of the Heavenly Host are said to dwell
Chesed (CHEH-sehd) - Home plane of the Fiends and other monstrous creatures. An incredibly dangerous place. The Fiends are considered the worst monsters in the Kabbalistic universe.
Geburah (ge-BOOH-rah) - The Death Realms, and infernal legion of Hell, according to some people. Most others just say it's the afterlife, the "Land of the Dead," where most spirits go. That doesn't mean everybody who dies goes here, but if they go anywhere other than Kether, it's most likely Geburah.
Tiphereth (tih-FEH-rehth) - A mysterious realm, even by Kabbalah standards. It's said to be a nexus of some kind, a way-station. It might also be the home of the Three Fates.
Netzach (NEH-tzach) - Home of the Gods. Where all the pagan lands are said to be: Olympus, Asgard, and the rest. Whether the Egyptian realm of Khem is here or in Geburah or somewhere else no one can say for sure.
Hod (HOHD) - The Dream Realms. Other than Malkuth, this is the only Sephiroth where every human goes, usually every night. Not everyone who dies goes to Geburah, but everybody dreams. People who dream are already at the Periphery of Hod. (Needless to say, the Aces' dream-journeys to places like the Cavern of Flame, Khem, and Tir Nan Og are very unusual and not at all like most people ever have. These are dreams, but also somehow more than that.)
Yesod (YEH-sohd) - The Faerie Realms. Since Arcadia is one of the Faerie lands, that means the gate in Inwood Park in northern Manhattan is actually a gate to Yesod.
Malkuth (mahl-KOOTH) - Earth, the world of humanity. In D&D terms, this is the "Prime Material Plane."
An important description of the Sephiroths was offered to Yukio Ohta and Cassandra Moon during their encounter with Python, Servant of Gaia at Delphi in 2004:
"“Kether, the source of pan-sentient existence. Binah, the realm of the true Natural Powers, those which emerge partly from the dreams of humanity but also from that which lies beyond them. Chokmah, the home of the architects. Chesed, the realm of change. Geburah, the land of that which comes after. Tiphereth, the gateway of the cosmos. Netzach, the land of the Powers Artificial, those whom humanity in this time creates and whom they mistakenly think most powerful. Yesod, my realm, the land of the Fae. Hod, the land of dreams. And Da’at the portal to realms beyond, to which my Sight cannot take me.”