Angels are created beings (Psalm 148:2, 5, Col. 1:16)–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).
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.
Angels were originally created for the purpose of serving and carrying out the will of God. The fallen angels rebelled and became evil angels. Satan is such an angel (Isaiah 14:12-16, Ezekiel 28:12-15).