A third possibility is to let the user move off into emptiness, letting them find out for themselves that they are lost. The second major area where navigation is a concern is when attempting to locate specific information in a large information system. A large information system can be as small as a book, or as large as all the information in the world. Planning a search for information over such a vast set of resources can be a daunting task. Real books have a table of contents, indexes, and other navigation aids.
Libraries have card catalogs.
Hyperlinks allow the user to jump to another part of the book where related information is located. Electronic media exists in far more forms than books and libraries. There are electronic versions of sound recordings and video data and there are thousands of computer databases and networks of computer resources. Overviews and Landmarks are two techniques which can help with the navigation problem. Overviews are like a book's table of contents.