Q: What does Judaism say about demons?
A: The Gemara speaks of them in a matter of fact way as quite real and entities to be reckoned with. The Mishnah states that they were created in the twilight of genesis (which is a subject of interpretation among the commentators). The Zohar also discusses demons, as do many other kabbalistic texts. Yet we have a tradition that the Arizal banished the demons of his era. And although some came back subsequently, the Baal Shem Tov to sent them packing. Since then — who knows?
Yet the Rambam, under the influence of the Arab-Greek rationalism of his day, basically dismissed them as imaginary, or at least inconsequential if one didn’t believe in them.
Many Modern Orthodox Jews today share the Rambam’s attitude, while Chassidim and other traditionalists believe in them, at least theologically.