Subject matter guides
Subject matter guides generally consist of a listing of online resources concerning a particular topic. They are usually created and maintained by subject matter experts. The portals and gateways to online tax information mentioned above are subject matter guides.
Web directories
Web directories are efforts to classify by subject matter all different types of Internet sites. Web directories are usually arranged as some sort of interconnected hierarchy consisting of several layers. Web directories enable users to search the web by subject matter. Some examples –
Yahoo
http://www.yahoo.com/
Open Directory Project http://dmoz.org
Search engines
Search engines (at least text-based ones) are vehicles that allow users to find Internet sites that contain particular text, words or phrases. The basic technology employed by just about every search engines involves, at its most simple level, a matching of query terms to terms in a document (with many, many complex variations on this single theme). Once the search engine has retrieved documents containing the text specified by the user, the search engine then ranks the pages or sites retrieved. The ranking mechanisms vary greatly among search engines, with sophisticated techniques being employed to ramk retrieved sites along different parameters. Meta-search engines are retrieval mechanisms that gather the results of a particular query from several search engines. The trend in search engine technology is to personalize (or at least allow manipulation of) results based on the individual user (or what the search engine can glean about the individual).
Examples of search engines:
Google
http://www.google.com/
Alta Vista
http://altavista.com/
HotBot
http://hotbot.lycos.com/
Teoma
http://teoma.com/
Examples of search engines for images:
Google
http://images.google.com/
Ditto
http://ditto.com/
Examples of meta-search engines:
Metacrawler
http://www.metacrawler.com/
SurfWax
http://www.surfwax.com/