Before we explore the world of search engine optimization, it is vital that you know a little about how search engines work and their relative market shares. It will help you to prioritize your activities later!

There are essentially four different parts to a typical large search engine; the crawler, the directory, sponsored results and the search engine itself.

Crawlers (e.g. Google) automatically visit web pages to compile their listings, making use of a so-called robot or spider (eg. Googlebot), which follows links from one website to another, ultimately compiling an index of all the pages and sites on the internet. These crawlers provide an index, which can then searched by the search engine. You may find that several or all of the pages on your site are indexed in thisway. Some search engines have their own crawler and others buy-in crawler results from others.

Human-powered directories, such as the Open Directory, rely on submissions from the public, which are reviewed by editors for inlusion in the directory. If you get included in a directory, generally only one page from your site (usually your home - or index - page) will be listed.

Crawled results are combined with sponsored results, supplied by pay-per-click (PPC) advertisers, and the results from human-maintained directories to complete the search engine index. Check out the Search Engine Reationship Chart at Bruce Clay inc. for the latest picture on who powers whom. You will note a couple of things right away. Firstly, the dominance of the Google and Yahoo! crawlers and secondly the importance of DMOZ directory results as a back-door for many search engines.

Search engines do not really search the web directly, but rather an index database of the full text of web pages, which itself is drawn from the billions of web pages on the internet's servers. Search engine databases are selected and built by computer robot programs called spiders.

If a web page is never linked to by any other page, spiders cannot find it, unless the (usually new) site is submitted manually by a human at the search engine's "add URL" page. All search engine companies offer ways to do this.

After spiders find pages, they pass them on to another computer program for "indexing." This program uses an "algorithm" to assess the text, links, and other content in the page for "key words" that might be searched on at the engine. This allows the search engine to order results served by their "relevancy" to the search terms used. As each search engine has a different algorithm, it will index sites in a different way and thus serve up different relevant results.

Some types of pages and links are excluded from most search engines by policy. Others are excluded because search engine spiders cannot access them. Generally, the use of frames, flash graphics and dynamic URLs all get in the way of effective spidering and should thus be avoided.

In addition to indexing pages, most algorithms seek to establish the "authority" of a site. A site which is linked to by many other sites (using keyword-rich anchor text) is assumed to be of greater merit than one with no links at all. This activity is called "ranking" and helps search engines to sort otherwise similar results into ever-more relevant and authoratative results.

Based on US analysis in January 2005, the top search engines (by share of total searches at home and work) are as follows:

Google Search - 47%
Yahoo! Search - 21%
MSN Search - 13%
All Others - 19%

These shocking figures do not convey the true dominance of the top players, as you have seen from the interdependence of search provision in section (a) above. You could be searching at AOL (part of the "other" 19%) and viewing Google results, for example.

There is also strong anecdotal evidence that Yahoo! and MSN tend to send more searchers through to their sponsored (or paid) results than do Google (due to the prominence of these results on their results pages). As such, for a typical small webmaster who does not use pay-per-click (PPC) advertising, they might get up to 80% of all their traffic from Google's various sites across the world.

Now you understand the market a little better, you will perhaps understand the obsession many webmasters have with Google! A top-10 position at Google for your key search terms can make your online business fly. If you drop out of that top-10, your business can literally collapse overnight!