Waterfall is one of the oldest project management methodologies. Unlike Scrum or Kanban, this model is not flexible because it prescribes a linear approach meaning each phase starts after each other. It actually means that all requirements, documents, project management plans are created in advance (during the first steps of the project) and they are not normally changed during the project lifecycle (anything actually can be changed, but change management process must be followed).
Depending on the project and domain, the phases can be different. However, original phases proposed by Waterfall's creator are the following:
It's important to understand that the phases can be changed depending on the project needs.
In order to manage projects using this methodology, you should determine steps/activities for the whole project (or phase) and implement these steps one after another (normally only in one direction, downwards). The goal is to determine all required steps to produce a product, service or result (it can also refer to a phase of project, not a whole project) and implement them.
Where Waterfall can be used?
The answer is easy. Everywhere. The methodology is especially popular in construction and some other industries. However, since Waterfall is not an iterative model, nowadays this is not so popular in some industries such as software development.
Disadvantages of the approach
In the real world Waterfall methodology is not the best one. Why? Because if you have a big project, the whole process can take a lot of time. Will the project be needed after this big period of time? Nobody knows. Can you plan all activities for the project which can take a couple of years? Yes, you can bit it can be very difficult. Businesses are change, requirements and values are change as well; that's why this methodology is not the best one to use.
Since the model requires a deep planning at the beginning, for some projects this is a good solution. With attention to documentation, clear structure, ability to quickly and easily understand the plan and set necessary milestones makes this methodology very powerful for experienced project managers.
Are there alternatives?
Take a look at Agile methodologies (e.g. Scrum). The model has many advantages if project manager does the right work (e.g. there is a proper change management, deep planning, etc.)