The Portfolios link navigates to where all portfolio items are displayed.
The Portfolio Center is a central list of Portfolio names used for grouping and organizing projects within your site app.
A Portfolio Item is a list item that can have multiple projects associated to it.
A Portfolio contains a set of Projects that may not all be related, but that are all associated to a strategic initiative or goal of the company. The benefit of using this app is that it’s a way to “tag” projects with this metadata classification to be able to view the project-related data and health at a higher level, such as by Portfolio. This is an optional feature and is not required.