The nine choirs of angels (seraphim, cherubim, thrones, dominions, virtues, powers, principalities, archangels and angels), are the ranks of the celestial hierarchy in Christian angelology, arranged in three triads of three. The seraphim, nearest to God, burn with love; the lowest choirs, the archangels and angels, are those most concerned with human affairs.
The ordering comes from the Celestial Hierarchy attributed to Dionysius the Areopagite, a work of around the fifth or sixth century, and was taken up by later theologians such as Thomas Aquinas, becoming the standard scheme of Western Christian thought about the angels.