[quote=Jannah]
The understanding I have of Jinn is that they're kind of like spirits or ghosts, which as we know tend to vary in form.
[/quote]

More like spirits than ghosts, in that they exist separate from humans, have their own souls/free will, and aren't simply imprints of the dead. :) They're very interesting. Also, of course, in Islam some of them play the role of "demons". Seeing as they have free will, just like us, some powerful, arrogant jinn have chosen to scorn Allah and lead mortals away from salvation, to their doom...

EDIT: There's no way for angels to rebel, as angels by definition have no free will, so the Corrupter-of-humanity figure is the first Jinni (the "Adam/Lilith" of Jinn), who grew arrogant, and turned away from the Creator.

EDIT 2: Meaning, in case it wasn't clear, that the first jinni is the equivalent of the Christian Satan, and the evil jinn are the equivalent of Christian demons, except possibly more tragic, in that they have free will, and they [i]chose[/i] to be the way they are...