Ophanim, also called Thrones or, less frequently, Galgalim, are an order of angels found in assorted angelic hierarchies and other religious apocrypha. They are described as two interlocking and overlapping chariot wheels, covered in eyes and burning with celestial fire. Ezekiel also refers to them as "having the gleam of a Tarshish stone," which may or may not be beryl.
In Ezekiel 1:15-21 during his vision, Ezekiel sees the Ophanim along side the Cherubim guarding God's throne. There, the Ophanim appear to be spiritually connected to the Cherubim, "(20) Wherever the spirit would go, they would go, and the wheels would rise along with them, because the spirit of the living creatures was in the wheels," which has led to some theological debate as to whether Ophanim are actually angelic beings, or if they are material things, or whether there are in fact two kinds of Ophanim/Thrones, one material and one celestial.
In Daniel 7:9, God's throne is described as being comprised of the burning wheels, which would imply that the Ophanim aren't just guarding His throne, they are the throne.
In the Christian Hierarchy of Angels, they are the 3rd rank down, beneath Cherubim and the Seraphim.
In the Mishneh Torah, Ophanim are second beneath the Cherubim (called "Living Creatures").
In the Zohar of Kabbalah, Ophanim are the 5th one down, beneath the Malakim, Erelim, Seraphim, and Chayot.