Catechism of the Catholic Church §414 — excerpt
Satan or the devil and the other demons are fallen angels who have freely refused to serve God and his plan. Their choice against God is definitive.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.