What are Archangels?

(Greek: ruling angel)

In its wider meaning, any angel of higher rank, thus all the higher orders of angels. Saint Michael, therefore, is called Archangel although he is the prince of the Seraphim. In its more restricted sense, the archangels are those blessed spirits who compose the second choir of the lowest order in the angelic hierarchy (see angel). As distinct from the guardian angels, the archangels are God's messengers to man in matters of graver moment, e.g., Gabriel to the Virgin Mary, Raphael to Tobias; and to the archangels God entrusts the care of persons of exalted rank or sanctity.

New Catholic Dictionary - O'Connell, The Holy Angels, New York, 1923 (A.J.S.)
You should be aware that the word "angel" denotes a function rather than a nature. Those holy spirits of heaven have indeed always been spirits. They can only be called angels when they deliver some message. Moreover, those who deliver messages of lesser importance are called angels; and those who proclaim messages of supreme importance are called archangels.