With evolution of ABAP Programming Model, now merely every application start with the CDS Modelling on the persistent layer. CDS(Core Data Services) model provides various models that can be used in different use cases. Here are below few of them-
Views are the old CDS views having a CDS entity and SQL view associated with it. View Entity is the successor of the view and don’t generate any ABAP SQL view during its activation. So use view entity instead of view.
Projection views are more of a specialization of the underlying view entities. It restricts the functionality of view entities.
Abstract Entities are used to define signature without implementation. In RAP(restful abap programming) this can be used to define data structures of the parameters and the result of actions and functions.