Yetzirah is the third of the Four Worlds, the World of Formation, the realm of the angelic choirs and of the formative forces, where the creative ideas of Briah are given pattern and structure. It is often called the astral or psychic world, the plane of the ruach, the spirit, and its sefirot are the six from Chesed to Yesod in the common correspondence. The name is shared with the Sefer Yetzirah, the Book of Formation, a short and enigmatic cosmological text redacted somewhere between the third and sixth centuries which describes the creation of the world through the ten sefirot and the twenty-two letters of the Hebrew alphabet; that book is the ancestor of much later Kabbalistic structure, though the four-world scheme itself is later than it.
The Sefer Yetzirah itself divides the twenty-two letters into three "mothers" (aleph, mem, shin, tied to the elements), seven "doubles" (letters with two pronunciations, tied to the planets and the days of the week) and twelve "simples" (tied to the signs of the zodiac and the months), a classification that later became one of the chief sources for the letter-planet-zodiac correspondences used throughout Western Kabbalistic and ceremonial magic. The world of Yetzirah takes the Tetragrammaton's Vav as its letter and, in the Golden Dawn's 1888 to 1900 London attributions, the element Air and its own colour scale.
Sources The ZoharTraditionally attributed to Moses de Leon with Sefer Yetzirah: The Book of Creation in Theory and PracticeAryeh Kaplan, The Golden Dawn: The Original Account of the Teachings, Rites, and Ceremonies of the Hermetic Order of the Golden Dawn, 6th editionIsrael Regardie