The waterfall model was the initial process model. It is also known as a linear-sequential life cycle model. It is pretty easy to understand and work. In a waterfall model, every phase must be completed thoroughly before the next stage can begin.
The projects which are small and there are no uncertain requirements use the waterfall model.
Unlike the agile model, in the waterfall model review take place at the end of each phase for determining the project is on the right path and to whether or not to continue or discard the project.
In waterfall model software testing starts solely after the development is complete. In waterfall model phases do not overlap.