For the ancient Semites they were descendants of disappeared ancient peoples who acted during the night and vanished with the first light of dawn; they could make themselves invisible or change shape into animals at will; these spirits were commonly made responsible for diseases and for the persecutory manias of some "lunatics", which they said were tormented by the jinns.
The Arabs used to believe that the jinns were spirits of fire, although sometimes associated them with beautiful women that visited men by night to copulate with them until they were exhaust, in this sense believing they extracted energy from the man in the same way vampires suck blood, weakening their victim.
See also houri.