About Search Engines
What is a Search Engine?
A search engine is a
program that searches documents for specified keywords and returns a list
of the documents where the keywords were found. Although search engine
is really a general class of programs, the term is often used to
specifically describe systems like Google and Yahoo that enable users
to search for documents on the World Wide Web and USENET newsgroups.
Typically, a search engine
works by sending out a
spider to fetch
as many documents as possible.
Another program, called an
indexer, then reads these documents and creates an index based on
the words contained in each document. Each search engine uses an
algorithm to
create its indices such that, ideally, only meaningful results are
returned for each query.
How Web Search Engines
Work
Search engines are the key
to finding specific information on the vast expanse of the World Wide Web.
Without the use of sophisticated search engines, it would be virtually
impossible to locate anything on the Web without knowing a specific
URL, especially as
the Internet grows exponentially every day. But do you know how search
engines work? And do you know what makes some search engines more
effective than others?
There are basically three
types of search engines: Those that are powered by crawlers, or
spiders; those
that are powered by human submissions; and those that are a combination of
the two.
-
Crawler-based engines
send crawlers, or spiders, out into cyberspace. These crawlers visit a
Web site, read the information on the actual site, read the sites
meta tags
and also follow the links that the site connects to. The crawler returns
all that information back to a central depository where the data is
indexed. The crawler will periodically return to the sites to check for
any information that has changed, and the frequency with which this
happens is determined by the administrators of the search engine.
-
Human-powered search
engines rely on humans to submit information that is subsequently
indexed and catalogued. Only information that is submitted is put into
the index.
In both cases, when you
query a search engine to locate information, you are actually searching
through the index that the search engine has created; you are not actually
searching the Web. These indices are giant
databases of
information that is collected and stored and subsequently searched. This
explains why sometimes a search on a commercial search engine, such as
Yahoo! or Google, will return results that are in fact dead links. Since
the search results are based on the index, if the index hasn't been
updated since a Web page became invalid the search engine treats the page
as still an active link even though it no longer is. It will remain that
way until the index is updated.
So why will the same
search on different search engines produce different results? Part of the
answer to that is because not all indices are going to be exactly the
same. It depends on what the spiders find or what the humans submitted.
But more important, not every search engine uses the same
algorithm to
search through the indices. The algorithm is what the search engines use
to determine the relevance of the information in the index to what the
user is searching for.
One of the elements that a
search engine algorithm scans for is the frequency and location of
keywords on a Web page. Those with higher frequency are typically
considered more relevant. But search engine technology is becoming
sophisticated in its attempt to discourage what is known as
keyword
stuffing, or spamdexing.
Another common element
that algorithms analyze is the way that pages link to other pages in the
Web. By analyzing how pages link to each other, an engine can both
determine what a page is about (if the keywords of the linked pages are
similar to the keywords on the original page) and whether that page is
considered "important" and deserving of a boost in ranking. Just as the
technology is becoming increasingly sophisticated to ignore keyword
stuffing, it is also becoming more savvy to Web masters who build
artificial links into their sites in order to build an artificial ranking.
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