What Are Search Engines and How Do They Work?
What is a search engine ?
Last Revised: 07/17/11
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contact us. One of our goals is to
help demystify search engine
optimization, and to make our clients
at least partially self-sufficient.
Toll Free: 1-877-691-8989
Local: 484-437-7977
Last Revised: 03/12/11
A search engine is a large and complex computer system that is used to find and index content
on the Internet. Google is the best known search engine. It is used to conduct roughly 65 percent
of searches world-wide. There are about 30 recognized, general purpose search engines, and
countless special purpose and single-site search engines across the Web as a whole.
Search engines perform three major functions:
- They find (spider or crawl) web pages.
- They inventory (index) page content.
- They capture user queries (keyword phrases), search their index, and rank order and display
findings (results).
A highly simplified 30,000 foot visual representation of these search engine functions is
displayed below. Notice two major streams of activity take place, on top and on bottom: (1) the
spider and index functions; and the (2) search function.
How Search Engines Work
(1) Spider and Index Functions
Computer programs called spiders or bots continuously crawl the web by following identified
links, page by page. When new and/or updated web page content is found, a search engine
database is populated (indexed) with every single word, every image, and every incoming,
internal and outgoing link to the page. Much more is captured, but these are the important parts.
Spiders return, on average, every four to six weeks to check for updated content. If site content is
updated more frequently, the spiders return more frequently. This is one reason why blogs are
popular. By maintaining a blog, you are "teaching" search engine spiders to return to your site
more frequently to check for new and updated content.
Not every page is indexed by a search engine. Spiders are relatively unsophisticated and can
sometimes have difficulty finding or traversing a website. Orphan pages (pages without any
inbound links) simply won't be found. Dynamic pages (constructed on the fly using parameters
input by human searchers) sometimes aren't indexed. Spiders can also be specifically
instructed to not index certain files and/or directories on a site using what are called robot meta
tags. These are all common reasons why some websites have little or no visibility on the web. If
you are curious, you can type your web page address into a spider simulation tool and see
exactly what a spider sees when it visits your web page. This is a good way to validate nothing in
your site design is impeding the indexing of your site.
In our example above, Page C is not stored in the index or database because there are no
inbound links to the page. A page must have at least one incoming link from an external site
before it can be found by a search engine spider.
(2) Search Function
In the bottom stream of our diagram, the third or search function occurs on demand when a user
wants to find information on the web. Note that search engines never actually search the web.
It would simply take too long. In reality, the search is conducted using the previously described
search engine index or database. The keywords in the search phrase are looked up in the index.
When a match is found, an algorithm is run to determine its relevance. The details of these
"ranking algorithms" are a closely held secret of search engine companies.
Using our Page C example again, the word "green" is the subject of a search. No match is found
because the page containing the word "green" is not captured in the search engine index.