By themselves, project goals have no substance, however clearly you specify them. Each goal is merely an intention, a wish floating on air. The way to crystallize and achieve your goals is to build a gridwork of solid beams beneath them. In your project, those solid beams are called tasks.
But simply including tasks in your project isn’t enough to reach your project goals efficiently. Individual tasks with no dependency on one another are like beams heaped haphazardly in a pile. Just as beams — or bricks or columns or arches — need to be grouped logically to be effective, so too do tasks. In a project, a logical grouping of tasks that completes a major step is called a phase. Often, completing a phase of a project indicates that a milestone, a significant event marking project progress, has been reached.
With Microsoft Project, you can easily group tasks into phases and specify milestones. Moreover, you can show project phases and subphases (and subsubphases, if you want), as well as the hierarchy among tasks by outlining your tasks. In Microsoft Project, a phase and any other logical grouping of tasks is represented by a summary task, which consists of and summarizes its constituent tasks.