Waterfall methodology is a sequential design process. This means that as each of the eight stages (conception, initiation, analysis, design, construction, testing, implementation, and maintenance) are completed, the developers move on to the next step. As this process is sequential, once a step has been completed, developers can’t go back to a previous step – not without scratching the whole project and starting from the beginning. There’s no room for change or error, so a project outcome and an extensive plan must be set in the beginning and then followed carefully.
Agile came about as a “solution” to the disadvantages of the waterfall methodology. Instead of a sequential design process, the Agile methodology follows an incremental approach. Developers start off with a simplistic project design, and then begin to work on small modules. The work on these modules is done in weekly or monthly sprints, and at the end of each sprint, project priorities are evaluated and tests are run. These sprints allow for bugs to be discovered, and customer feedback to be incorporated into the design before the next sprint is run. The process, with its lack of initial design and steps, is often criticized for its collaborative nature that focuses on principles rather than process.