Waterfall and Agile are the two well-known approaches that software development companies use to organize their software development lifecycle activities.
Similar to the way water cascades from the top downward in a waterfall, the waterfall method follows a sequential or linear process where one phase is completed before the next one starts. However, in practice, phases may overlap to reduce effort and time by starting some activities in one phase before the previous phase ends.
Agile, on the other hand, is an iterative and incremental approach to software development lifecycle activities where the solution evolves as the team improves their understanding of the requirements.
There are a several implementations of Agile. Scrum, Kanban, Extreme Programming (XP), DevOps are all methods within the family of Agile. While each method differs in its approach to the development lifecycle, they all emphasize continuous iteration of work processes. Additionally, they all aim to deliver value to customers quickly.
Each method is best suited for different types of projects. The waterfall approach works well when upfront planning is needed. Agile, on the other hand, is more suited to projects with a high degree of uncertainty and a high rate of change, complexity, and risk.
This means that Agile can be more susceptible to scope creep and therefore cost overruns, whereas the more structured nature of Waterfall can provide more predictability and consistency but can be less responsive to change.