This is the type of software process model in software engineering that takes all the fundamental process activities of specification, development, validation and evolution and represents them as a separate process stages.And by the process stages it means all the requirement specification, software design, implementation and testing and so on. As we can see the figure below for the waterfall model, it clearly explains a lot about the software development life cycle. And as defined by the tasks, it is also known as ‘software life cycle’. It is a good example of plan-driven process, as everything that we do is only after planning and scheduling. The main stage directly reflects the essential development activities.
Based on the principle, every phase in this model is documented out and need to be approved for the next phase. One cannot start working on the next phase until and unless the documents of previous model is approved out. It is like this process involves the feedback from one phase to another. Documenting helps to monitor the problem and progress. This model is adopted only when the requirements are well understood and does not need to be changed during the development phases.