In Christian mythology, an angel came to mean a benevolent spirit created by God. In Judaism however, it meant literally a messenger of God which could apparently be applied to both human messengers or supernatural messengers depending on the context. The Hebrew word for angel means "messenger", as does the Greek word ἄγγελος (ahn-geh-los) the word from which the English word is ultimately derived. In both mythologies and a few others, there is more or less a hierarchy of angels, although they are not the same and have occasionally changed depending on the author. Many of them only existing in extra-biblical literature.
They perform various tasks, like delivering messages from God to man, acting as executioners and warriors, and protecting humans from harm, and telling God that he is holy.