Maimonides writes that everything which G‑d created may be divided into three categories: those things which are a composite of mutable substance and form; those of immutable substance and form; and those creatures which are form without substance. The latter are the angels. They are not bodies nor physical beings, but rather forms which are separated from one another by the principles which they represent. Thus, “form” in this case does not mean dimensional structure, but rather a clearly defined spiritual principle or permutation of divine energy. Angels...are separated from one another by the principles which they represent
Angelic beings fall into two general categories: those which were created during the six days of Creation (referred to as the “ministering angels”), and those which are created on a day-to-day basis to fulfill various missions in this world. The names of the angels change in accordance with their mission.
When angels are sent down to this world as emissaries of the Holy One to carry out a specific mission, they become clothed in a body formed from the element of air or of fire. Angels may also appear in human form. Once they have completed their worldly duties, they divest themselves of their bodies and return to their former spiritual state.
There are angels which inhabit each of the four worlds: Ofanim in the world of asiyah, chayot in the world of yetzirah and seraphim in the world of beriah. There are also angels in the world of atzilut.
The angels are further subdivided according to the sefirah from which they derive. The angels are thus divided into “camps” according to the division of the sefirot. The angel Michael and his “camp,” for example, derive from the sefirah of chessed; the angel Gabriel and his “camp” derive from the sefirah of gevurah; and so on.
For a deeper look at angels: Angels 1: The True Story.