KPhotoBook is an easy to use, intuitive and fast application to manage and organize your photos. The main idea is to be able to annotate your photos with tags (these tags are arranged in a tree) and to refind the photos using the given tags.
Sorry, KPhotoBook is dead. No releases for almost two years, no changes in CVS for 1,5 years. I suggest you try media-gfx/kphotoalbum which is well-maintained and does the same and more.