The most direct definition or explanation in Scripture concerning angels is found in the first chapter of Hebrews.
"To which of the angels said He at any time, Sit on My right hand, until I make thine enemies thy footstool? Are they not all ministering spirits, sent forth to minister for them who shall be heirs of salvation?" (Hebrews 1:113-14).
The word "ministering" in this passage is translated from the Greek root, "leitourgeo," and means to serve, or to perform a service. Leitourgeo is used in only two other passages in the New Testament, both referring not to angels, but to believing humans.
"As they ministered to the Lord, and fasted, the Holy Ghost said, Separate Me Barnabas and Saul for the work whereunto I have called them" (Acts 13:2).
"For it hath pleased them of Macedonia and Achaia to make a certain contribution for the poor saints which are at Jerusalem. It hath pleased them verily; and their debtors they are. For if the Gentiles have been made partakers of their spiritual things, their duty is also to minister unto them in carnal things" (Acts 15:26-27).
In essence, angels are servants of servants. The fulfill a role of great humility in God's eternal economy (although they are at times very forceful in the exercising of their ministering role). They are not to be exalted or overly emphasized, therefore, as the writer of Hebrews so plainly reveals. A Biblical understanding of their existence and function leads to much appreciation for these servants of both God and ourselves, but in relative terms, angels are clearly meant to be behind the scenes players in the fulfillment of our Heavenly Father's eternal purpose in Christ.
As are we. Perhaps the greatest lesson to us concerning angels is the quiet, in the shadows way in which they serve. I love the passage in Isaiah 6, wherein the seraphim above the throne of God are said to cover their faces with 2 of their wings (Isaiah 6:2). There may be numerous reasons for this, but one is surely to direct attention away from themselves, and unto the face of the God they love and revere. Our own person should be hidden in such manner, to the degree that the Lord's trusting saints are not to even let their left hand know the good works of their right (Matthew 6:3). Tragically, we live in a generation in which prominence, fame, and accompanying power and wealth are considered acceptable throughout a Christendom that seems to forget that our Lord Himself lived 90% of His life in complete obscurity (while the other 10% was lived publicly in a manner that always deflected attention to the glory of His Father - John 14:10).
Angels are servants of servants. So are we. "By love, serve one another" (Galatians 5:13). May the quiet and always unseen service provided to us by our fellow citizens of "the whole family of Heaven and earth" be an example, encouragement, and challenge to us as we cover our faces in order to shine all attention on the face of God.
"Not unto us, o Lord, not unto us! But unto Thy name give glory for Thy mercy and Thy truth's sake!"