The Church teaches the existence of demons. What are demons?

Demons, like the angels, are spirits: immaterial and bodiless, but evil spirits which oppose God and seek the spiritual destruction and annihilation of man, though it is not uncommon to find situations in which they bodily torture man and make man's life on earth sheer tyranny within that measure of freedom allowed them by God. And God allows them this freedom either for the punishment or chastisement of the sinner, or for the trial of the righteous. The demons were not created by God as demons: evil and unclean spirits. In the beginning they were brilliant angels-just as all the heavenly spirits. However, they were overcome and dominated by arrogance and pride.

Thus, under the leadership of their capitan Lucifer, Eosphoros (lit the light-bearing angel), who subsequently became Satan, they revolted against God. Lucifer tried to become like God in all things by domineeringly usurping divine authority and glory. "I will go up above the clouds", thought Lucifer, "I will set my throne above the clouds. I will I will be like the Most High" (Isaiah 14, 13-14). For this reason, God cast him down into Hades, and with him all those angels who believed and followed him. All these became demons, evil spirits, and their leader became the Devil, Satan, the " father of lies", as our Lord so names him. The Tempter, the Dragon of Revelation, the ancient Serpent, who appeared to Eve as a serpent in order to lure her into disobedience and sin, Belial, Beelzebub-as he is called in the New Testament- the Seducer who seduces the universe, possessing deadly hatred for God and blind passion against man. His demons are like him.
Of course there do exist people who deny the existence of demons, people who say that the Devil does not really exist. "And what is the Devil?" they ask. "He is merely the personification of evil". Yet practically every page of the New Testament speaks about Satan and mentions so many people who were possessed by demons and from whom our Lord cast them out. The Evangelist St. John emphasizes that "for this purpose the Son of God was manifested, that He might destroy the works of the Devil" (I John 3, 8). It is indeed sad when people and especially Christians deny this truth, so abundantly witnessed to by Holy Scripture. Indeed, such a denial could prove both disastrous and fatal. To deny the Devil's existence is to fall into one of Satan's very own traps, set in order to trap with all certainty his victims, and to lure them first into sin and then to cast them into perdition.
 

That the demons at their creation were free and immaterial angels and that they made ill use of their freedom, becoming dark and evil, "their uncleanness not being attributed to their creation," that "they are not evil by nature, but good, having been created with good intent and for a good purpose, and having no trace of evil implanted within them by the Creator, but possessing freedom of choice and the power to remain with God or to separate themselves from the good, and that they did not remain where God had created them but reviled Him and fell from heaven", are truths taught by all the Fathers of the Church. Indeed, St. Athanasius underlines the type of evil responsible for Lucifer's being cast out of heaven and transformed into Satan. He clearly and lucidly states that Satan was not cast out from heaven "because of fornication, adultery or theft", but because of his arrogance and pride. And Cyril of Alexandria verifies that the Devil "was banished from the heavenly courts" "because he dared to say: I will be like the Most High, and because he "imagined that he could" though a creature, "ascend to the nature of the Creator". To become "co-ruler with Him Who has the dominion over all", as St. Basil states.