O-oh, y-y-yeah, I h-heard you the first time. Ghosts. W-why do you want to know about them?
On their own, ghosts are quite harmless. They are generally not strong enough to have an affect on things around them, besides scaring the hell out of anybody who happens upon them.
They can scream, wail, knock around light objects, and cause incredible illusions, but they are ultimately unable to reach out at the living.
Necromancers often use ghosts to learn a great deal about the world.
Contrast with geists, however...