In your account you have access to 7 different applications: Calendars, Files, Photos, Bookmarks, Discussions, To Dos, and Addresses. Each of these applications (depending on your account and the application) can have at least one Folder Set and multiple Folders. The Calendar application does not have folders or folder sets but can have multiple calendars.
If you have applications or Folder Sets that you don't use you can hide them from your Navigator on the left-hand side of the screen using the "Manage Applications" dialog. Follow the steps in the image below:
If you un-check all of the boxes in an application, the entire application will be hidden from your Navigator. You can always go back to the “Manage Applications” dialog to turn the application back on.
When you turn Folder Sets, Calendars, or applications off in you account, it does not delete any of the information contained in any of these items. Turning them off only hides them.