Monsters Incorporated is the largest scare factory in the monster world, and James P. Sullivan (John Goodman) is one of its top scarers. Sullivan is a huge, intimidating monster with blue fur, large purple spots and horns. His scare assistant, best friend and roommate is Mike Wazowski (Billy Crystal), a green, opinionated, feisty little one-eyed monster. Visiting from the human world is Boo (Mary Gibbs), a tiny girl who goes where no human has ever gone before.
my two cents:
my understanding of this movie: unseen spirits from another realm cross into the human dimension to feed off the energy of human beings using fear and manipulation tactics.
monsters = demons, spirits, etc.
monster world = realm or dimension