It is mass-produced food. It is often pre-prepared and delivered to the shop ready for frying or boiling. There is a strong priority on "speed of service".
The restaurants that sell fast food are called "fast food shops" or "fast food restaurants". Some of the more common fast food restaurants are McDonald's, Wendy's, Burger King, Taco Bell, and KFC. All these chains have a limited short menu, and serve the same kind of food in all their outlets. The shops are subcontracted by the supplying company, and tied absolutely by contract to that single supplier. However, the same kind of criticism could be (and is) directed at some other kinds of restaurants. The problem is seen by many critics as being one of over-eating, rather than one of the specific food types.