The idea that evil is powered by fear is a primary underlying theme in many horror tales (see Stephen King's It for example). Monsters, Inc. breaks this myth by illustrating that evil, in the form of monsters, might not be what we think, and that there's something more powerful than fear that could be used to power their world.
The focus of Monsters, Inc. is a made-up world of monsters, Monstropolis, which is powered by the screams of little children. Since not all children have the same fears, monsters (and children) are carefully profiled such that the scream output is maximal. A child not afraid of a monster is therefore particularly terrifying to them.
Every night, the monsters venture into our world (through closet doors) and collect screams, with a running competition as to who collects the most. The scream leader is Sully (voiced by John Goodman), who's actually a good-hearted sort, and in second place, vying for first, is Randall Boggs (Steve Buscemi), who's evil even by monster standards.
One child, Boo (Mary Gibbs), who apparently isn't afraid of anyone except Randall, enters the monster world and causes panic. It turns out that Boo is a victim of a more global conspiracy that Sully and his friend Mike (Billy Crystal) must unearth. In doing so, they find a source of power that is more fulfilling than a child's scream.
The film contains some great animated sequences, including a chase scene involving thousands of doors. As with many other animated movies, the gadgets and workings in the power factory are imaginative and inspiring. Monster's Inc., like Shrek, is an animated film made for both young and old alike. While it doesn't delve too much into the realm of parody, it's novel story line combined with quality animation and voices make it a must-see on the big screen.