The Waterfall model was the first process model to be introduced. It is very simple and easy to use. The key to the waterfall model is that each phase has to be completed before the next phase can be started. At the end of each phase a review is done to decide whether to go ahead with the project or to scrap it all together.
The advantages of the waterfall model is that it is quick and easy to use and understand. The model also works well with small projects where the needs are well known.
The waterfall model also has its disadvantages like once an application is in its testing stage it is difficult to go back and change something even though the concept might not be well thought out. The waterfall model is not a model for a long, ongoing and complex project that has a risk of changing.
The waterfall model is best used when
1) The project is short.
2) There are no ambiguous requirements and when the needs are clear.
Here is an example of a waterfall model