Waterfall model:

The waterfall model is a sequential software development model (a process for the creation of software) in which development is seen as flowing steadily downwards (like a waterfall) through the phases of requirements analysis, design, implementation, testing (validation), integration, and maintenance

Usage of the waterfall model:

In Royce's original waterfall model, the following phases are followed perfectly in order:
1. Requirements specification
2. Design
3. Construction (aka: implementation or coding)
4. Integration
5. Testing and debugging (aka: verification)
6. Installation
7. Maintenance

To follow the waterfall model, one proceeds from one phase to the next in a purely sequential manner.For example, one first completes "requirements specification" — they set in stone the requirements of the software. When the requirements are fully completed, one proceeds to design. The software in question is designed and a "blueprint" is drawn for implementers (coders) to follow — this design should be a plan for implementing the requirements given. When the design is fully completed, an implementation of that design is made by coders. Towards the later stages of this implementation phase, disparate software components produced by different teams are integrated.After the implementation and integration phases are complete, the software product is tested and debugged; any faults introduced in earlier phases are removed here. Then the software product is installed, and later maintained to introduce new functionality and remove bugs.