Angel means messenger. Angels are created (Psalm 148:2, 5; Col. 1:16)--non-human, spirit beings (Heb. 1:14). They are immortal (Luke 20:36), innumerable (Heb. 12:22), invisible (Num. 22:22-31), sexless (Matt. 22:30), and do the will of God (Psalm 103:20). These angels have a ministry to believers. They guide (Gen. 24:7, 40), protect (Psalm 34:7), and comfort (Acts 27:24).
There are good angels (Gen. 28:12; Psalm 91:11) and bad angels (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).