The "visible web" is what you see in the results pages from general web search engines. It′s also what you see in almost all subject directories. The "invisible web" is what you cannot retrieve ("see") in the search results and other links contained in these types of tools.
数据挖掘研究院
- Searchable Databases. Most of the invisible web is made up of the contents of thousands of specialized searchable databases that you can search via the Web. The search results from many of these databases are delivered to you in web pages that are generated just in answer to your search. Such pages very often are not stored anywhere: it is easier and cheaper to dynamically generate the answer page for each query than to store all the possible pages containing all the possible answers to all the possible queries people could make to the database. Search engines cannot find or create these pages. More explanation.
- Excluded Pages. There are some types of pages that search engine companies exclude by policy. There is no technical reason they could not include them if they wanted. It′s a matter of selecting what and what not to include in databases that are already huge, expensive to operate, and whose search function is a low revenue producer. More explanation.
