The Ghosts are the supporting antagonists of the horror movie, Poltergeist - angry and confused, these souls latch onto the life force of a young girl known as Carol Anne after having their original gravesite disturbed by the careless actions of Lewis Teague and are further controlled by "the Beast" (an alias of Reverend Henry Kane) into causing havoc on the mortal plane.
The ghosts never show themselves in person but manifest in many frightening ways, most notably with poltergeist-activity (hence the name of the film): they also begin to employ much more dangerous and vivid tactics including actual kidnap.
The ghosts also create several "minions" to do their bidding - such as the Clown Doll and the Gnarled Tree, though Reverend Henry Kane likely had some input in these events due to his exceptional power as a demon.
In the end of the movie, the ghosts take it upon themselves to finally put an end to the torment by literally spewing forth the contents of the gravesite while also causing the Freeling's home to implode and thus purify their sacred site again: destroying a significant portion of the nearby community in the process. These spirits achieved their own retribution in the end.
But lately, another group of spirits among the ghosts who are completely and they simply followed the will of the Beast. In fact these spirits are the losts of a Satanic, 19th century utopian cult who will be still after Carol Anne.