There are differences amongst the Angels, but we do find the origin of these differences in Revelation.
It is open to discussion if the Angels are a same species, or if there are as many species as there are choirs, or even if each individual Angel is a species in himself (St. Thomas Aquinas’s opinion).
According to a tradition that comes to us by Dionysius – the Pseudo-Areopagite, the theologians group the Angels in nine orders, or, choirs, divided in three hierarchies (the names were taken from the Sacred Scriptures):
First hierarchy: Seraphim (Seraphs), Cherubim (Cherubs), Thrones;
Second hierarchy: Dominions, Virtues, Powers;
Third hierarchy: Principalities, Archangels, Angels.
The Angels of the three first choirs, or, first hierarchy – Seraphs, Cherubs and Thrones – continually contemplate and glorify God: “I saw the Lord sitting upon a throne, high and lifted up… Above Him stood the seraphim… And one called to another and said: "Holy, holy, holy is the LORD of hosts” (Is 6, 1-3). “The LORD reigns… He sits enthroned upon the cherubims" (Ps 98, 1).
The three choirs of the second hierarchy – Dominions, Virtues and Powers – rule the world. Finally, the last three choirs – Principalities, Archangels and Angels – execute God’s orders: “Bless the Lord, all ye His Angels: you that are mighty in strength, and execute His word, hearkening to the voice of His orders” (Ps 102, 20).
All of them are Angels at God’s disposal to execute His wishes. Although the Gospel, in the Annunciation to Mary, mentions the Angel Gabriel (Lc 1, 26), it does not mean that he belongs to the last of the Angelic hierarchy, since the sublimity of his embassy leads us to suppose that he is amongst the foremost spirits who assist God.
The three so called Archangels – Saint Michael, Saint Gabriel and Saint Raphael – probably belong to the highest angelic hierarchy.
Although we do not know the exact number of existing Angels, we know, through the Sacred Scriptures, that their number is very great. That is what can be found in the book of the Apocalypse: “…and I heard the voice of many Angels round about the throne...and the number of them was thousands of thousands.” (Revelation 5, 11). And in the book of Daniel: “a thousand thousands (Angels) served Him, and ten thousand times ten thousand stood before Him" (Dan 7, 10).
Many theologians say the number of Angels is superior to the number of men that existed since the beginning of the world and that will exist until the end of times. The reason given by Saint Thomas for this is - since God has always sought mainly for a perfect Universe, when He created all beings - the more perfect a class of beings, the more prodigal God was in creating them. Since the Angels are much more perfect than men, they were created in a greater number.