Software development life cycle involves many design models; Waterfall Model, V-shaped model, spiral model and Incremental model are the three main types of SDLC models which are popularly used. RAD (Rapid Application Development) and Prototype Models are also part of SDLC process.
One can only use Waterfall model if the requirements are already known to the developer, unless it is useless. It follows same steps including Requirement, Design, Coding, Testing, Implementation and Maintenance. Before proceeding towards next step, it is necessary to complete the previous phase properly. It is fast and easy to use model.
This model is similar to Waterfall model except that the main importance is given to the testing phase in this model and all the phases are implemented in sequential manner only. System plan is made before starting the development phase and instead of focusing on writing the code, testing methods are decided first.
Also known as iterative method focuses mainly on the larger portion of the software and focus on decorative parts afterwards. The idea behind this model is that due to completion of important and bigger part first, the probability of having errors and bugs will be reduced.