The word "angel" comes from the Greek word "angelos" meaning messenger and is used 294 times in Scripture. Angels are also referred to as hosts (angelic or heavenly) Gen. 2:1; 32:2; principalities & powers (Eph. 3:10); sons of God (Job 1:6; Gen. 6:4)
There are different orders or class of angels:
Seraphim - only mentioned two times in Isaiah 6:1-7;
Cherubim - worshiping angels. Cherubim look like men except that each one has four faces. Their general appearance is like fire and lightning and move with the speed of lightning. They are connected to wheels, having a firmament (floor of showing solidarity) over their heads. Above this is a throne which God sits. They are celestial beings who guard and vindicate the righteousness of God. They guard the way to the Tree of Life (Ezek. 1:5-28; 8:1-4; 10:1-22; Gen. 3:24); Zoa - or living creatures, are similar to cherubim except only one head each and are full of eyes before and behind. Like the seraphim in that they have six wings. Worship angels (Rev. 4:6-9)
Spirit horses and chariot riders (2 Kings 2:11-12; 6:13-17; Zech. 1:8-11)
Archangels - Arch means "chief, ruler or leading angel". There is only one truly mentioned in scripture as an archangel, and that is Michael. Michael appears to be the Commander of angels, a chief angel. He is also one of the chief princes or rulers (Dan. 10:13; Jude 9; Rev. 12:7-9; 1 Thess. 4:16)
Angelic beings - or common angels (messengers). There are approx. 300 scriptures to do with angels (Dan. 10:12-14).
Angels are innumerable (Rev. 5:11; Heb. 12:22; Job 25:3); have inconceivable power (2 Chron. 32:21; Gen. 19:1, 12-13; Matt. 26:53); power, position and authority to execute God's judgments (Matt. 13:41-42; Rev. 7:1; Rev. 8-13, 9:1, 9:13-14, 10:1-7); intelligent and wise, patient; meek; glorious; immortal; holy; travel at blinding speed; can appear invisible and visible; are joyful and need no rest.
As guardian angels/protectors - Matt. 18:10; Psa. 91:11-12; Acts 12:5-10; Dan. 6:22; 2 Kings 6:8-17
As ministers - to attend, to aid, do things needful or helpful (Matt. 4:11; Gen. 24:7; Exod. 23:20; Psa. 104:4; Heb. 1:14)
We read in Hebrews 13:2 that there may actually be times when we are confronted by angels without realizing it:
"Be not forgetful to entertain strangers, for thereby some have entertained angels unawares."
Angels have the ability of stepping into our physical realm of existence, but as spirit beings they predominantly move about and exist on a plane that mostly goes unnoticed to the naked eye. Unlike angels that do possess a body (spirit bodies are of a different makeup than ours) demonic spirits are disembodied spirits that are only able to manifest physically through possession of a living host (thus the term "demonic possession"). Some confuse demonic spirits as "fallen angels" but they are not for Scripture defines them as separate. The origins of demonic spirits are not for certain and opinions vary, but what is certain is that they serve and follow the dictates of Lucifer, now commonly known as the devil or Satan.