The Waterfall model constitutes the traditional project managememt method in software development: In the analysis and requirements phase, requirements are collected and specified in detail. In this (and only in this) phase the participation of the later users is intended. Afterwards, implementation takes place in successive project phases, whereby a subsequent phase can only start when a preceding phase has been fully completed. This method is also called waterfall model because these project phases are graphically represented as bars arranged in a cascade, which reminds of a waterfall.
Waterfall models can be used in projects where requirements and processes can be described very precisely and remain stable. In this case, user involvement via iterative feedback/improvement loops is not necessary (compare Agile Project Management). The waterfall model typically allows for a reliable estimation of costs that are comparably easy to meet due to the stable requirements. The use of Waterfall models is not recommended for projects with unclear/fuzzy requirements. Critics of that method point out that due to the lack of involvment of end users during the development phase via iterative processes, errors are detected late and must then be corrected with considerable effort.
