Restless souls. You just have to reap them.
Well, obviously, it's more complicated than that.
Ghosts have variations to their abilities, decided by their cause of death. Normal properties include: being able to phase through objects, posessing objects, and having difficulty being perceived. Emotional distress caused by their own death changes their behavior, and if not immediately, eventually. They tend to drag others to their death.
To describe perceptibility, living beings can't see souls and cloaked reapers without a few conditions. They have to experience near death or we have to constantly interact with them. Interaction over time makes living beings have an easier time spotting us, but it opens them up to being affected by ghosts' powers. It's not worth messing with occult stuff.
Defeating ghosts is a cakewalk. They're weak to salt, silver, incense (especially copal), and our scythes. Sometimes they need to be soothed with offerings or fulfilling their regrets. Also very easy. But unless they're reaped, they'll continue to remain, so leave it to reapers.