Here is a tool that is extremely useful for SEOs to know, it is provided by BruceClay.com and is called the Search Relationship Chart. (pdf here) They even have a histogram with the relationships as they have changed since November 2000. It outlines the suppliers and receivers of primary and secondary search results (raw index) and paid results (ads).
What is so important about the relationships between search engines?
Believe it or not, Google does not own the Internet. The Internet is a community, yes, I will have to say that Google does command a big stake in this community, but they do not own it all!
Based upon the demographics of the people you are trying to reach, they may not think so either. Knowing you users is an important aspect of any marketing plan and you shouldn’t discount other sources of potential traffic, namely, everyone else.
Ranking high in one engine does not guarantee great rankings in all of the engines, if you need proof, search for your top keyword in each of the big four and look at the top ten, are they all the same? Typically not.
So, what’s my point?
Don’t put all of your eggs in one basket. You may be missing out on valuable traffic if your focus is only on Google or any other engine alone. It is this reason that a balanced, holistic SEO campaign is necessary.
If a website ranks #1 for a search term and no one uses it (the search term) does it even matter?
Building a website for ranking will do only one thing, bring more traffic. Great, but are you providing what these users are looking for? Bridging the gap between getting them into your website and them actually doing what you want are two very different things. Traffic does not mean sales, just the opportunity.
Build your website for the users you are trying to reach.
Oh, and the chart, I would recommend that you print it and keep a copy on your wall (or cubicle wall) that you can reference easily.