Royce's Waterfall Model

The unmodified "waterfall model". Progress flows from the top to the bottom, like a waterfall.

In Royce's original waterfall model, the following phases are followed in order:

1. Requirements specification
2. Design
3. Construction (AKA implementation or coding)
4. Integration
5. Testing and debugging (AKA validation)
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, which are set in stone. 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.

Thus the waterfall model maintains that one should move to a phase only when its preceding phase is completed and perfected. Phases of development in the waterfall model are discrete, and there is no jumping back and forth or overlap between them.

However, there are various modified waterfall models (including Royce's final model) that may include slight or major variations upon this process.

http://en.wikipedia.org/wiki/Waterfall_model