The spiral model is a software process model that couples the iterative nature of prototyping with the controlled and systematic aspects of the linear sequential model. Barry Boehm mentioned the Spiral model in this paper (1986).

A prototype model is a toy/demo implementation of the actual product or system. A prototype model usually exhibits limited functional capabilities, low reliability, and inefficient performance as compared to the actual software.

In Iterative waterfall model, the feedback paths are provided from every phase to its preceding phase. In practice, it is not possible to strictly follow the classical waterfall model for software development work. In this context, we can view the iterative waterfall model as making necessary changes to the classical waterfall model so that it becomes applicable to practical software development projects.

The classical Waterfall Model was the first Process Model. It is also known as a linear-sequential life cycle model. It is very simple to understand and use. Classical waterfall model is the earliest, best known and most commonly used methodology.