Talking about the Rapid Application Development (RAD) aspect of a product such as Delphi, we surely have to start with the TComponent class. The main concepts of RAD are based on the idea of pluggable, reusable, and configurable building blocks combined to implement complex applications easily while keeping the developer in an affordable complexity zone.
Back in the early days, it was easy (and quite popular) to divide Delphi into two big layers – namely, RTL and VCL. VCL was (and still is) a collection of components built to wrap Windows controls in a Delphi-friendly interface, adding uniformity for many functionalities across different components (that is, font settings, positioning, and alignment settings). It also enables the possibility to have them in the Delphi IDE at design time, allowing developers to actually visually design their applications (dragging components from the tool palette to a form or a frame and editing their properties...