The idea that the cast of the movies was in some way "cursed" is a superstition based on these facts: four of the stars on these movies died deaths that can be characterized by the credulous as mysterious or tragic. It is not clear that this particular film differs from others in the number or nature of the deaths of its actors.
Various urban legends surrounding the film have been attributed to the potential cause of the curse. The most widely blamed is that allegedly real human skeletal remains were used as props in the first film, causing the angry spirits of the deceased to wreak havoc.
The Poltergeist series is just one of a handful of movies and series that have been said to have been "cursed". Some other movies and series include Rebel Without a Cause, Diff'rent Strokes, The Crow and In The Heat Of The Night (TV series).
In Poltergeist's case, the actors and actresses who died include: