A Sheyd/Sheydim are entities, most often serving “wicked” purposes. While “demon” is the most common English translation, Judaism has a different relationship with its “evil” creatures and spirits. Everything serves a purpose in our universe and everything is made/connected to the divine.
Sheydim have wings like angels, are able to fly, and obtain secret knowledge. They eat, sleep, and procreate like humans. They are not immortal, but are long lived. They are distinguishable by their bird-like feet and invisible to most humans.
Sheydim are vulnerable to iron. Amulets like hamsas, relevant biblical verses, numeric codes, and the names of the divine are also used to defend against Sheydim. Pious behavior and safety in groups (3 or more at night) is usually the best defense.