Purpose built lightweight race cars. The lower weights usually use motorbike engines and the top level are custom built. Radical are the most famous manufacture for that group of cars with the RS8 being the most well known due to its Nordschleife record. But some other manufactures are Caterham, Y.O Concept, Norma Auto Concept. For racing they feed into Le Mans and prototype cars more than the GT type do. Also the most affordable type of racing if you want to get into real racing cars rather than modified road cars. They are used on track and hillclimb.
Oh cool, I guess we will have the Honda 2&4 to fit into that group haha