What are the Holy Angels?

 
The angels are immaterial, spiritual and immortal beings. They do not posses a material body such as ours but are spiritual beings and therefore invisible. They cannot be seen, just as our soul, which is also a spirit, cannot be seen. God created the angels before He created the material word. This derives from the words of God who said to Job: "When the stars were made, all my angels praised me with a loud voice" (Job 38, 7).
That the angels are a creation of God is clearly stated in Holy Scripture: "by Him were all things created, that are in heaven, and that are in earth, visible and invisible, whether they be thrones, or dominions, or principalities or powers" (Col. 1, 16). Thrones, dominions, principalities and powers are names of various angelic orders.
As immaterial and spiritual beings the angels transport themselves with ease from place to place; however, they are not omnipresent like God, for they, too, are creatures of God with limited abilities and powers. Though their powers are much greater, indeed, incomparably greater, than those of man, they are nevertheless found infinitely lacking when compared to God's omnipotence. Their knowledge is also great. Much greater than human knowledge; yet it cannot be compared with God's omniscience. Their wills were tested when Lucifer rebelled against God, and through divine grace their will has become firmly rooted in good, towards which it continuously inclines. It is for this reason that they are called the Holy Angels.
At times-whenever God so willed- they have been sent to holy men in order to reveal to them God's will. At such times they took on the form and appearance of young men, occasionally winged, and spoke in human tongue, otherwise it would have been impossible for them to announce to man God's will.
  According to the Blessed St. John the Damascene, "an angel is an intelligent essence in perpetual motion, with free will, incorporeal, ministering to God, having obtained by grace an immortal nature". Hence, the angels are spirits-pure spirits-immaterial, heavenly, bodiless and immortal. For this reason, they are also called heavenly and bodiless powers, and are much superior to man, both in knowledge and in power; yet they are neither all-wise nor all-powerful, for only God is such. The angels are divided into three hierarchal orders: Seraphim-Cherubim-Thrones
Dominions-Powers-Authorities
Rulers-Archangels-Angels.
St. John the Damascene further states concerning angels: "They are in perpetual motion, with free will, having obtained by grace an immortal nature". "They are", he continues, "rational and intelligent, and endowed with free will, immortal not by nature but by grace... They are not hemmed in by walls and doors, bars and seals. When they are in heaven they are not on earth, and when they are sent by God down to earth they do not remain in heaven". Three angels are referred to by name in Holy Scripture: Michael, the leader of the people of Israel; Gabriel, the Angel of the New Testament and Raphael. Of the three, two are Archangels.