The waterfall model is a sequential design process for software development consisting of conception, initiation, analysis, design, construction, testing, production, and maintenance phases. Each phase must be completed fully before the next begins, flowing steadily downwards like a waterfall. It is simple to understand and implement for any size project but lacks flexibility, making it difficult to go back and change something once a phase is complete. Requirements gathering and analysis is the first phase where requirements are documented and understood before being broken down and designed in subsequent phases.