A significant amount of valuable information on the Web is generated from databases. In fact, it has been estimated that content on the invisible Web may be 500 times larger than the content provided by search engines. People sometimes referred to them as specialty databases, subject-specific databases, virtual libraries, and other similar terms. As Web technology develops and greater amounts of information are mounted on the Web, these databases take on primary importance as information finding tools. This can include large listings of things with a common theme.
All directories are part of the deep Web.
Examples include phone books, lists of professionals such as doctors or lawyers, patents, laws, dictionary definitions, geographical information, items for sale in a Web store, etc. A good directory will link to database sites on the Web. This is because many of these databases have Web sites of their own. For example, you can go directly to the Moreover site and browse its datatabase of current news stories. A directory, therefore, can link to the Moreover site from its news section.