(14) Now the spirit of the LORD had departed from Saul, and an evil spirit from the LORD terrified him.
(ו) עשרה דברים נבראו בערב שבת בין השמשות, ואלו הן: (1) המזיקין, (2) וקבורתו של משה, (3) ואילו של אברהם אבינו. (4) ויש אומרים, אף צבת בצבת עשויה.
(6) Ten things were created on the eve of the [first] Sabbath at twilight, And these are they... Also the destructive spirits, And the burial place of Moses, our teacher, And the ram of Abraham, our father. And some say: Also the [first human-made] tongs, made with [Divine] tongs.
Rabbi Geoffrey Dennis
Demons are spirits that act malevolently against human beings. The Bible makes repeated mention of evil spirits. Apocalyptic literature offers the first attempt to explain their existence in a monotheistic context by claiming demons are really fallen angels.
Rabbi Dennis on the Ibn Ezra
Described demons as a product of the interaction of sunlight with smoke and vapor which then clings to the body, causing illness (Sefer ha-Atzmim).
Chagigah 16a
They resemble angels in three ways: They have wings, they can fly throughout the universe, and they hear what transpires in heaven. They also resemble mortals in that they procreate, eat, and die. They are always invisible, except under special conditions.
The malevolent effects of demons are many: they cause illness and death, especially for the vulnerable (children, women in childbirth); they trouble and deceive the mind, and cause contention in the community of mortals.
Megillah 3a
The appearance of demons varies, but is always terrible. In keeping with Ancient Near Eastern beliefs about evil spirits, demons have bird talons for feet in addition to wings. At night, demons can appear in human form
Demons
by Rabbi Geoffrey W. Dennis
http://www.pantheon.org/articles/d/demons.html
Dybbuks, demons and exorcism in Judaism
Surrounded by invisible forces
http://www.jewishjournal.com/cover_story/item/dybbuks_demons_and_exorcism_in_judaism_20120627
Dybbuk or Demon: Knowing Your Jewish Ghosts and Ghouls
http://www.tor.com/blogs/2012/10/dybbuk-or-demon-knowing-your-jewish-ghosts-and-ghouls