The document discusses the waterfall model, which is a sequential software development process where progress flows steadily from one phase to the next - conception, initiation, analysis, design, construction, testing, production/implementation, and maintenance. The key phases and deliverables are completed one at a time before moving to the next phase. The waterfall model is simple and easy to understand, manage, and works well for smaller projects with well-defined requirements. However, it is inflexible and carries high risks since changes are difficult once a later phase has begun and no working software is produced until late in the lifecycle. The model is not suitable for complex, long-term, or ambiguous projects where requirements may change.