The sidebar on the left side of the Shotwell window lists various views of your library. Although you may see the same photo in multiple views, it's only stored once on your hard drive.
The Last Import view lists your last imported photos no matter if they are imported from F-Spot, your camera, a memory card or the hard disk.
The Flagged lists all photos you had previous flagged.
The Saved Search allows you to sort your libary by many criterias.
The Events folder lists all events in your library. An event is a group of photos that were taken at approximately the same time.
The Tags folder lists all tags you've assigned to photos. Photos can have multiple tags attached to them. When you click on the name of a tag in the sidebar, you will see all the photos associated with that tag.