The term, angel, i.e., "messenger," is used of God, of men, and of an order of created spiritual beings whose chief attributes are strength and wisdom.
In the Old Testament the expression "the angel of the Lord" (sometimes "of God") usually implies the presence of Deity in angelic form. In the New Testament the word "angel" is used of men. In Rev. 8:3-5 Christ is evidently meant. Sometimes "angel" is used of the spirit of man.
Though angels are spirits, power is given them to become visible in the semblance of human form (as men). The word is always used in the masculine gender, though sex, in the human sense, is never ascribed to angels.
They are exceedingly numerous. Their power is inconceivable. Their place is about the throne of God. Their relation to believers is that of "ministering spirits, sent forth to minister for them who shall be heirs of salvation," and this ministry has reference largely to the physical safety and well-being of children of God. It appears that this care for the heirs of salvation begins in our infancy and continues through life. The angels observe us, a fact which should influence conduct.
Man is made "a little lower than the angels," and in the incarnation Christ took "for a little [time]" this lower place that He might lift the Christian into His own sphere above angels.
The angels are to accompany Christ in His second advent. To them will be committed the preparation of the judgment of individual Gentiles among the nations. The Kingdom Age is not to be subject to angels, but to Christ and those (us) for whom He was made a little lower than the angels. An archangel, Michael, is mentioned as having a particular relation to Israel and to the resurrections. The only other angel whose name is revealed, Gabriel, was employed in the most distinguished services.
In regard to fallen angels, two classes are mentioned: 1) "The angels who kept not their first estate [place], but left their own habitation" and are chained under darkness, awaiting judgment. And 2) the angels who are not bound, but go about doing the will of Satan. They may be identical with the demons. Everlasting fire is prepared for Satan and his angels.
I hope this quick summary doesn't raise more questions than it answers, which it seems to me that it does. Nevertheless, I love studying the angels and plan to write in more detail later.