Below is a very short and simple differenciation between the two types of Buildings, one could go into much detail but really it starts with the simple difference.
'Modular Building Construction' is a term used to describe the use of factory-produced pre-engineered building units that are delivered to site in Modules. The modular units may form complete rooms, or parts of rooms.
Modular buildings are generally made into six sided boxes constructed in a factory, then delivered to site and using a crane, the modules are set onto the building's foundation and joined together to make a single building. The modules can be placed side-by-side, end-to-end, or stacked, allowing a wide variety of configurations and styles in the building layout. Typically in South Africa Modular Buildings are referred to as "Parkhomes".
So what is a Prefabricated Buildings then:
A prefabricated building, informally a prefab, is a building that is manufactured and constructed using prefabrication. It consists of factory-made components that are transported and assembled on-site to form the complete building.
So its quite simple Prefabricated Buildings are very different to Modular buildings. But then how do many of these companies not actually know what they are selling? Well I will leave the conclusion up to you to make for yourself!