Yes, 六道轮回. There are 6 rebirth paths : gods, humans,
demi-gods, animals, ghosts and hell. It is not easy to be born as
humans, so cherish your human body and practice according to
Buddha's teachings and stop the cycle of reincarnation because you
do not know which path you will be reborn in your next life.
Those wondering spirits on earth, which category they belong
to?
I know of some encountering spirits disturbing them day and
night, don't these spirits have any source in charge of them to
control them?