The basics

Projects have a hierarchical structure. The top-most line is the project's main line, which contains general data. Below this line are the project lines:

The project lines may be different in nature and may be on multiple levels. Together they form the structure of your project, which could, for example, consist of subprojects, activities, milestones etcetera:

When setting up a project, there are various terms and types of project lines you should be familiar with. For example, milestones and activities are used to register different things. These basics are explained here.