At the start of any project, there must be a defined end goal of what is to be achieved. There must then be a clearly defined path to achieve that goal. A project manager can lay out a timeline with deliverables to be met at certain intervals, which are the milestones.
Each project will have different requirements for the deliverables that need to be completed by the milestone dates. The types of projects can be process-based, a phased approach, product-based, or a critical change.