The Waterfall Development Methodology is a linear design approach that goes through a defined set of stages (like a waterfall).
This Methodology maintains that you should only move onto the next phase once the previous phase is reviewed and verified (no going back)
The waterfall model provides a structured approach, the model progresses linearly and is easily understandable and explainable to clients and has clear milestones in the process.
The waterfall is not flexible but has a fixed scope and is clearly understood