Rapid Application Development(RAD) is the parallel development of software functionality and then integrating those functions.

In Rapid Application Development(RAD) functions/components are developed in parallel into time-boxed cycle and then all these functions are assembled into a working prototype.

By RAD customers gets something working very quickly and can provide feedback for the software or the future requirements. Since the customer gets early visibility in the software he can provide early feedback on the design and can decide whether to proceed with the development or not. Also he can suggest what functionality he needs in future releases.