Angels are created beings (Psalm 148:2, 5; Col. 1:16). They are non-human spirit beings (Heb. 1:14). They cannot die (Luke 20:36), cannot be numbered (Heb. 12:22), are normally invisible but can manifest themselves to us (Num. 22:22-31), have no gender (Matt. 22:30), and do the will of God (Psalm 103:20). Angels guide believers (Gen. 24:7, 40), protect them (Psalm 34:7), and comfort them (Acts 27:24). There are also elect angels (1 Tim. 5:21). They were created without gender (because that is a biological function) but occasionally appear as males in Scripture (Genesis 19:12-13).
There are also different kinds of angels: seraphim (Isaiah 6:3), cherubim (Ezekiel 41:18; Genesis 3:24; 1 Samuel 4:4), archangels (1 Thessalonians 4:16; Jude 9), rulers and powers (Ephesians 6:12), and fallen angels (Revelation 12:3-4).
Angels perform different functions as revealed in the Scriptures such as announce the birth of Jesus (Matthew 1:20-21), deliver the law (Acts 7:53; Hebrews 2:2), gather the elect (Mark 13:27), gather the wicked at the end of the age (Matthew 13:39-41), guard the tree of life (Genesis 3:24), minister to people (Genesis 16:7-10), and return with Christ at his second coming (Matthew 25:31; Mark 8:38).
Some angels are good (Gen. 28:12; Psalm 91:11), and some are evil (2 Pet. 2:4; Jude 6). The only angels mentioned by name are Gabriel (Dan. 8:16; 9:21), Michael (Dan. 10:13, 21; 12:1), and Lucifer (Luke 10:18). Michael is always mentioned in the context of battle (Dan. 10:13) and Gabriel as a messenger (Luke 1:26). Of course, Lucifer, who became Satan, is the one who opposes God.
God created for the purpose of serving and carrying out his will. The fallen angels rebelled and became evil angels. Satan is such an angel (Isaiah 14:12-16; Ezekiel 28:12-15). The first mention of an angel is in Gen. 16:7 where it is referred to an angel of the Lord: "Now the angel of the Lord found her by a spring of water in the wilderness, by the spring on the way to Shur.," (Genesis 16:7). In the New Testament, the Greek word is ἄγγελος, angolos and it occurs 177 times.