In short: Anything that gets buried in the "Pet Sematary" (misspelled by children that way), a former Indian burial ground, returns undead and with a vicious nature. A family from the city moves there, first their cat gets run over, is buried and returns evil (but not really dangerous) and then their small son. The father, in his grief and against warnings that what barely works for pets should not be attempted for humans, buries his son there who returns alright and starts slashing up neighbors and the family ...
Neither the cat nor the kid would ever eat anything again after their return from the burial grounds.
B movie that it was, it's probably King's best and most faithful screen adaption. (Shining was excellent in other ways, but had little to do with the book.)