The Waterfall Methodology
The waterfall model is a sequential software development model, a process for creating 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.
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 are combined 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.
The Agile Methodology
The catch line for Agile software development is develop quickly, deliver often. There are many agile development methods; most minimize risk by developing software in multiple repetitions (or 'iterations') of short time frames (known as 'timeboxes'). Software developed during one unit of time is referred to as an iteration, which typically lasts from two to four weeks. Each iteration passes through a full software development cycle, including planning, requirements analysis, design, writing unit tests, then coding until the unit tests pass and a working product is finally demonstrated to stakeholders. Documentation is no different from software design and coding. It, too, is produced as required by stakeholders. An iteration may not add enough functionality to warrant releasing the product to market, but the goal is to have an available release (with minimal bugs) at the end of each iteration. At the end of each iteration, stakeholders re-evaluate project priorities with a view to optimizing their return on investment.
http://en.wikipedia.org/wiki/Waterfall_model
http://en.wikipedia.org/wiki/Agile_software_development
http://agileintro.wordpress.com/2008/01/04/waterfall-vs-agile-methodology/
http://www.youtube.com/watch?v=GcR-wpSzr4Y
http://www.hyperthot.com/pm_sdm.htm
