Monsters, Inc. is the power utility in Monstropolis, a monster-populated city. The company uses the power of childrens' screams when scared by its employees, entering their rooms via closets linked to special doors on the company's "scare floor". The screams are used to generate Monstropolis's power. The company has recently had trouble harnessing enough power, due to the increase in children watching scary movies; that means that they're less susceptible to Monsters, Inc employees. 
 
Eventually, Sulley    becomes president of Monsters, Inc., and changes the company approach. They learned that a child's laughter is ten times more effective than their screams, so they now send monsters in to make children laugh.