Prefab refers to a general term that encompasses all sorts of prefabricated building methods. Anything built separately in a facility or factory then shipped to the construction site would be considered prefabricated. Modular buildings are a type of prefab, but they are built and shipped in self-contained units that resemble building blocks and are their category entirely. For reference, all modular buildings are prefab buildings, but not all prefab buildings are modular buildings.