Search Engines
They do not search the WWW directly, instead a "spider" crawls web pages
and indexes them and then creates a database. It is advantageous if you
wish to have a great variety of results, e.g. if you search the
temperature of a specific place, a search engine would be good place to
start. Most search engines use page ranks for their results based on
different factors.
Subject Directories
Directories created by humans instead of a computer.
Its database is indexed by editors. Due to that, there is not as a great
variety of results as common search engines. It is beneficial if you
search for a specific topic, e.g. American History.A sub directory has a
hierarchical structure.
Metasearch Engines
Do not have its own spider or database. Uses all the existing
equipment. Gives you results from various search engines. Provides fewer
results than one large search engine. It is a good place to start in
situations where you would wish to have different results from various
sources. This gives you the opportunity to contrast and compare your
search and extend it beyond one search engine's results.
Academic Databases
Usually include academic journals, which you mostly have to sign up for. Those search engines provide you with academic material. Furthermore, they are part of the invisible web which cannot be accessed by crawlers.
http://prezi.com/ej7y9p7assfc/search-tools/
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