Agile Testing can start at the beginning of the project with a continuous alliance between testing and development. Agile Testing is continuous but not sequential. The agile team works towards a common objective of achieving Quality as a single team. Agile Testing has shorter time frames from one to four weeks which is called iterations, release, or delivery driven approach. The motto of agile testing is to give a better prediction of the functional products in short duration of time.
Waterfall Model is very easy to understand and use. In this model, each phase completed before the next step begins and there is no overlapping in the stages. In the Waterfall approach, the whole software development process divided into separate phases. This model is sequential which means after completion of the previous stage the next stage begins. It is also known as a Linear-Sequential Life Cycle Model due to its representation of the software development process in a subsequent linear flow.
The agile method used when very limited planning required in a short time with change. The scrum method used when you have to complete any complex task in a fixed deadline. Waterfall method used when the requirements are very well known, clear and fixed with stable product definition.
Scrum is a useful agile process for product development, especially software development. It applies to the project with complex needs, aggressive deadlines. It helps the development team to concentrate on all phases of the product like quality, performance, usability and much more. Scrum has a short fixed plan of release cycles with adjustable scope known as sprints. Each project and release have multiple Release Cycles and sprint respectively.