Each search engine has it's own special series of filters to provide the best possible answer to any given question. But ultimately, each will select an html file. And so we can see that there is a common denominator which we can focus on and which melts away any difference between each engine. HTML.
This is why near all SEO information you locate begins to teach you HTML. And this gives you your first clue as to what an SEO expert is.
Essentially, each search engine builds it's own index based on your HTML code. This often includes other data such as the time it takes to load any resources attached to your page, if the content makes sense according to html rules, who else links to it, who else copies it etc.
Google, Bing and Yahoo are generally considered to be the top three engines. And each of these engines has arrived through different beginnings. Google earned it's fame for developing a brilliant search engine, Yahoo for it's excellent web directory and Bing because it's owner is a popular apple inspired Operating System. Common estimates as to percentage of usage are 82, 12 and 5 respectively. As you master and apply the SEO techniques you learn through out this course, you yourself will discover the difference as it applies your website. I will hazard a rough guess that near 98% of your search engine referrals will come from Google.
The real and full answer will be locked within the secrets and agenda of each engine's algorithm.
But the SEO can ponder over this:
If you offer excellent value on your web site, and if any search engines are not finding you, then they are not finding other sites that would offer you excellent value either. Make sense? Then why support or use an engine that does not support you?
Recently (2010) Bing implemented a new beaut algorithm which promises to revolutionize search results. Yahoo has jumped on board with it and now derives it's core search results from Bing as of 2010. And soon you will be able to monitor exactly how effective each search engine in sending you traffic so you will know first hand the effectiveness of each.
Anyone is allowed to create a search engine. There was a great little one-man developed engine called hotbot not too long ago (it is no longer the same, but it was very impressive). With the amount of pages that come on line every day, we might even see a market for single purpose search engines to evolve in popular markets, like a food only search or a houses only search for example. That would require a separate set of unique seo style rules.
If the day ever comes when no search engines seem to be able to find you, start a new search engine because the world will be ready for a new number one in the field. Start by using the keywords "meta" tag in the HTML Head. That is what made Yahoo great in the beginning.
For those of you that enjoy a bit of a laugh, Matt Cutts (resident google seo guru), upon hearing Microsoft steadfastly denying that Bing serves any Google results to their client's search queries, decided to post a couple of rather random and unique comparison Google Vs Bing search results for you to draw your own conclusions...