Via Inside the Vatican Magazine:
Angels are spirit beings, messengers and servants of God. Only four are ever named in the Bible: Gabriel, Michael, and Raphael (Raphael appears in the deuterocanonical book Tobit (initially accepted by both the Jewish and Christian canons, but removed from the Jewish canon in late antiquity and rejected by the Protestant reformers in the 17th century.
The word angel in English is a fusion of the Old English word engel (with a hard g) and the Old French angele. Both derive from the Latin angelus, and from the Koine Greek angellos ("messenger"), used in the Septuagint (the Greek translation of the Hebrew Bible done in about 200 B.C.) to translate the Hebrew mal'akh (yehowah) "messenger (of Yahweh)".
Angels have a special role in major events of Jesus’ life, and Jesus referred to them. Gabriel appeared to Zechariah to announce that he would have a son, John the Baptist (Luke 1:11-19); Gabriel also told Mary that she would have a son, Jesus (vv. 26-38). Jesus said that "little ones who believe in me" have angels in heaven who care for them (Matthew 18:6, 10).
Angels rejoice when people turn to God, and they bring the righteous to paradise (Luke 15:10; 16:22).
The earliest known Christian image of an angel, in the Cubicolo dell'Annunziazione in the Catacomb of Priscilla in Rome, is dated to the middle of the third century. It is without wings. But by the late 300s, angels were regularly depicted with wings.
St. John Chrysostom explained the significance of angels' wings this way: "They manifest a nature's sublimity. That is why Gabriel is represented with wings. Not that angels have wings, but that you may know that they leave the heights and the most elevated dwelling to approach human nature. Accordingly, the wings attributed to these powers have no other meaning than to indicate the sublimity of their nature."
Jesus also said that the devil has "his angels" (Matt. 25:41). These are more commonly called demons, or evil or unclean spirits. The chief demon is Satan (which means "the adversary"), also called the devil (one who leads others astray), Beelzebul (lord of the house), the evil one, the enemy, the tempter, or the prince of this world.
In a parable, Jesus described himself as tying up Satan and taking his possessions (Matt. 12:29). He spoke of seeing Satan fall (Luke 10:18). Through his death on the cross, Jesus drove Satan out (John 12:31-32). Satan was condemned (John 16:11). Jesus predicted that victory would be complete at the end of the age (Matt. 13:39-42; 25:41).