Topical coverage on the Invisible Web is extremely varied. This presents a challenge, since it is impossible to anticipate what might turn up in a database. In addition, this coverage will be fluid as databases proliferate on the Web. Information that is dynamically changing in content will appear on the Invisible Web. Examples include news, job postings, available airline flights, etc.
Directories are a part of the deep Web.
Subject directories can be highly useful when you have a broad topic and wish to view recommended sites relevant to that topic. Search engines are appropriate to use when you are looking for a specific site or have a narrow topic to pursue. It is one of the Web's largest commercial portals. But it is not a reliable or adequate research tool and should not be used for this purpose. It is very helpful to understand the principles of Boolean search logic when using a search engine on the Web.