The innate possession of intelligible species is common to all angels and is characteristic of their nature. But all angels do not possess the same species; and here we are coming to the basis for distinguishing among them. What constitutes relative superiority among created beings is their greater or lesser proximity to the first being, or God.
The first hierarchy contemplates intelligible essences in God himself. Now, God is the end of every creature. Accordingly, angels in this hierarchy contemplate as their proper object the highest end of the universe, which is the goodness of God. Those who behold it most clearly are called Seraphim, because they are as it were on fire with love for this object which they know so perfectly. The other angels in the first hierarchy contemplate the divine goodness, not directly and in itself, but under the aspect of providence. They are called Cherubim, that is, fullness of knowledge, because they see clearly the pristine operative power of the divine model of things. Immediately below the preceding come the angels who contemplate in itself the disposition of the divine judgments. Since the throne is the sign of judicial power, they are called Thrones. This does not imply that God's goodness, essence, and knowledge, by which he knows the disposition of beings, are three distinct realities in him. There are simply three aspects under which those finite intelligences who are angels can behold his perfect simplicity.
The second hierarchy does not know the reasons of things in God himself as in a simple object, but in the plurality of universal causes. Thus its proper object is the general disposition of means in view of their end. Now, this universal disposition of things presupposes the existence of many directors; these are the Dominations, whose name indicates authority because they prescribe what other angels must carry out. The general directives issued by the first angels are received by others who multiply them and channel them according to the various effects to be produced. These angels are called Virtues, because they confer on the general causes the energy required to carry out their numerous operations without fail. This order, then, presides over the activities of the entire universe, so that we can reasonably ascribe to it the movement of the heavenly bodies, which are the universal causes from which come all the particular effects that take place in nature. To these spirits, too, apparently belong the carrying out of those divine effects outside the ordinary course of nature and that are often immediately dependent upon the influence of the stars. Finally, the universal order of providence, already at work in its effects, is preserved from all disorder by the Powers, whose task is to safeguard it from all those baneful influences that might possibly disturb it.
With this last class of angels we approach the third hierarchy, which knows the order of divine providence, not in itself, nor in its general causes, but as it can be known in the multiplicity of particular causes. These angels are placed in immediate charge of the administration of human affairs. Some of them are especially appointed to the common good and the general welfare of nations and cities. Because of their dignity they are called Principalities. The distinction of kingdoms, the transference of temporary supremacy to one nation rather than to another, the leadership of princes and great men belong directly to their ministry. Beneath this very general order of goods comes one that effects both the individual taken by himself and, under the same title, a great number of individuals. Such are the truths of faith that must be believed, and divine worship that must be respected. The angels, whose proper object is these goods, which are both particular and general, are called Archangels. They also bring to people the most solemn messages from God. It was the archangel Gabriel who came to announce the incarnation of the Word, the only Son of God, a truth everyone is obliged to accept. Finally, there is a still more particular good that concerns every individual in himself and as an individual. In charge of this order of goods are the Angels properly speaking, the guardians of men and women and God's messengers for less important announcements. With them, we reach the end of the lowest hierarchy of separated intelligences.
Extracted from ‘Thomism, the Philosophy of Thomas Aquinas’ by Étienne Gilson (5th edition). P. 201-202