Concise Bible Dictionary:

It is to be regretted that the translators of the AV did not use the word “demon” where these words occur instead of “devil,” for which there is another Greek word, διάβολος, signifying “accuser.” This latter word is used only in the singular, referring to the devil—Satan; but there are many demons. Philosophers spoke of demons quite differently from the way they are represented in scripture. Thus Plato says, “Every demon is a middle being between God and the mortal. God is not approached immediately by man, but all the commerce and intercourse between gods and men is performed by the mediation of demons.” This was a device of Satan, that God could be worshipped through the agency of demons or demi-gods. In a similar way the Roman Catholics pray to the Virgin and the saints to intercede for them. Scripture makes it plain that the demons were evil spirits (compare Rev. 16:13-14).
Scripture also shows that idolatry was essentially demon-worship, the idol itself being nothing. “They sacrificed unto demons (shed), not unto God” (Deut. 32:17; 1 Cor. 10:19-20); “they shall no more offer their sacrifices unto demons (sair)” (Lev. 17:7; Rev. 9:20). Jeroboam had fallen so low as to have ordained priests for the demons (sair) and for the calves which he had made (2 Chron. 11:15); and some had “sacrificed their sons and their daughters unto demons (shed)” (Psa. 106:37). The things worshipped may have been unseen objects, or they may have had some mystical representation, or may have been mere idols; but behind all these were real beings, evil, unclean spirits; so that it was morally impossible to have fellowship with the Lord Jesus and with these demons (1 Cor. 10:19-21).
The evil spirits that possessed so many persons when the Lord was on earth were demons, and from the instances given we learn much respecting them. The Pharisees said that the Lord cast out demons by Beelzebub the prince of demons. The Lord interpreted this to mean “Satan casting out Satan”; by which we learn that the demons were the agents of Satan; and that Satan as a strong man had to be bound before his kingdom could be assailed (Matt. 12:24-29). The demons also were strong ones, by the way they handled those they possessed, and by one overcoming seven men and making them flee out of the house naked and wounded (Acts 19:16). We know also that they were intelligent beings; for they knew the Lord Jesus and bowed at once to His authority. They also knew that punishment awaited them: for some asked if the Lord had come to torment them before the time (Matt. 8:29).
It must not be supposed that demon-agency has ceased: the exhortation is, “Believe not every spirit, but try the spirits whether they are of God: because many false prophets are gone out into the world” (1 John 4:1). With this agrees the declaration that “in the latter times some shall depart from the faith, giving heed to seducing spirits and doctrines of demons” (1 Tim. 4:1). Spiritualists and Theosophists carry on intercourse with such, and are taught by them. In a future day also, when God will be pouring out His judgments on the earth, men will not repent, but will worship demons and all sorts of idols (Rev. 9:20). The spirits of demons also, by working miracles, will gather the kings of the earth together to the battle of that great day of Almighty God (Rev. 16:14). And mystical Babylon will become “the habitation of demons, and the hold of every foul spirit, and a cage of every unclean and hateful bird” (Rev. 18:2). The world and the professing church are evidently ripening for these things; and some, under the plea of investigating phenomena, are unconsciously having to do with the wicked spirits themselves!