Angels are rational beings, the highest beings in all of creation. They have no body, but are pure consciousness. They, however, are not God, for God is Being Itself, whereas angels are beings in creation. Gos is totally Simple, meaning His essence is HIs existence, and thus He is His intellect and the object of His knowledge. An angel’s intellect is a faculty of his nature.
Moreover, angels’ knowledge differ from ours in the sense that we come to know things. A baby has the potential to know much more than he or she does currently. Angels, on the other hand, have known all they will ever know since the moment God created them. We thus say that angels have no potency for knowledge, whereas we do.
Angels are in choirs, meaning ranks. The higher ranking an angel is, the simpler He is, meaning He has a greater semblance of the Lord, and vice verse the lower ranking (and closer to us) they are. Angels also know in an all-seeing gaze. They do not some to know things in steps. If an angel had to solve a difficult math problem, it would see it and immediately know the answer, with no intermediary steps.
Angels are those who chose God and thus received, and enjoy forever, the Beatific Vision. Demons are those who rejected God and thus suffer forever in Hell. Lucifer among the seraphim, the highest rank of angels, but he rejected God and thus fell. But how did this choosing or rejecting the Lord take place?
St. Thomas explains that God comes to His end without any motion, for He is His end, whereas we come to our end (God) by many movements, often not toward God (when we sin). But angels come to God by one action, not by many movements like us. Moreover, this act is a purely spiritual act, rather than a corporal act, for they are pure consciousness.
Angels have free will. They chose either God or sin, and thus were beatified or damned by one action.
Christ died to save mankind, but not angels, for angels did not need or cannot have redemption. Angels have only chosen good. Demons have chosen only evil, and they have turned their back on God forever.
What was the sin? It must have pertained to the spirit, not to the flesh, since angels have no bodies. St. Thomas postulates that the angels’ sin was either that of pride or envy, as they could have thought they could become like God on their own, or they envied another angel’s higher position, or they loved themselves above God. We do not know exactly what the angel’s sin was, nor will we ever until, we pray, get to Heaven.