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. Subject areas are condensed into short descriptions and hyperlinks can be used to get to the more detailed information. Overviews can be hierarchical and span vast quantities of data.