”The Qabalah describes the universe as divided into four separate “Worlds”, each of which is represented by one letter of the Divine Name, or Tetragrammaton (יהוה).
The first is Atziluth, the Archetypal World, the world of Pure Spirit which activates all of the other worlds which evolve from it. Here are attributed the Gods of the Sephiroth, and the letter Yod, Primal Fire.
The second world is Briah, the Creative World, the level of pure intellect, of the Archangels and of Heh, Primal Water.
The third is Yetzirah, called the Formative World because here are found the subtle and fleeting patterns behind matter. It is the realm of angels and of Primal Air, symbolized by the letter Vau.
The final World is Assiah, the active world containing both the physical world of sensation and the unseen energies of matter. It is the realm of the Cherubim, the final Heh of the Divine Name and Primal Earth.”
– The Qabalistic Tarot: A Textbook of Mystical Philosophy