Production Managers are hired by the producers, they take care of the production aspects of the film, ie. planning and managing schedules, resources, negotiating, dealing with clients and other managers. The UPM is the problem solver for the entire production process.
The unit production manager (also known as the UPM or just Production Manager) is the chief administrator of any film shoot. They're ultimately responsible for the film's budget, the film's schedule, and the general managing of the controlled insanity of a film set.