Search engine, search system, search site, search index or search directory ?

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If I understand well my sources, search engine, search system, search site and search index are quite synomyms.
Soooo, Google can be called a search engine, a search system, a search site or a search index, just as MSN/Bing or Yahoo!
I propose We use the term "search engine" to designate the four synonyms. KISS (1).

A search engine is builded based on a huge database, spread on a large number of computers.
Periodically, softwares, they call them spiders or robots, "travel" accross the web, copy and index information
about websites they "meet" in the database : this is called 'spider crawling'

And that's the heart of the question ! They don't simply copy and index all pages of websites, but according to
(secret) algorithms, they grab only a piece of it, a piece they find relevant to store (mainly 'words in pages' - title,
subtitle, meta tags, links,  ... - and 'where the words were found')

That's why some sites are referenced on the database, and some are not.
And that's also why some referenced sites appears at the top of the list, others at the middle at in the end.
That what they call 'SERPs' : Search Engine Results Pages

So, to be referenced is one thing. To be well 'SERP referenced' (top 10) is another piece of cake...

We need to learn more about these algorithms to build our sites efficiently.
What do spiders search on a website ? What do they "think" relevant ?
What can we do to make our sites relevant for robots ?

A search directory seems to be another structure. It contains information about web sites and not about their web pages.
Soooo, Open Directory Project (www.dmoz.org) is clearly a search directory. Google and Yahoo!, both known mostly for their search engines capabilities, have
also a search directory 'dark side', which can be accessed by dir.google.com and dir.yahoo.com.

Search directories dot not use spiders or robots to collect information about web sites.
Information, like title or description of the site, is submitted by the site owner.

FACTS
* dir.google.com doesn't create the directory itself; it gets it from the Open Directory project
* The two most important directories are Open Directory and Yahoo!
* Google's name is a variation of the word "googol," which is a mathematical term for a one followed by 100 zeros

RELATED ARTICLES
* How Internet Search Engines Works, by Curt Franklin

(1) Keep It Simple & Stupid