A task in a project is a work package that can be planned, assigned, executed, and reported. Tasks are a fundamental element of project management and are used to break down complex projects into smaller, manageable pieces.
A milestone in a project is a significant event or achievement that marks the completion of a major deliverable. Milestones serve as a way to measure progress and provide visibility into the status of a project.
Sub-projects are a way to organize and structure complex projects. A sub-project is a project within a project and can be used to manage separate parts of a larger project. Sub-projects can be used to manage different phases of a project, different locations, or different departments.