What are projects?
You organize your originals and versions using projects, which are similar to events in iPhoto. When you import photos into Aperture, you assign them to a project. You can create as many projects as you like, up to the limitations of your disk space. For example, you can create a new project for each of your shoots. Or, if you do several shoots of the same subject, you can create a project that encompasses all of the shoots.
You can have multiple projects open at the same time, each represented by its own icon in the Library inspector.