SEO or Search Engine Optimization means a process or technique used to make the quality or the websites better so that they get friendly to the Search engines, and the website gets traffic through the search results by having better position in the results in search engines. The higher the ranking of a website for keyword is, the more traffic it gets. And the traffic is considered to be organic or natural.

SEO Companies all around provide SEO Services like providing one-way-links, contextual links, directory links, Social bookmarking, Article marketing etc. to get the client’s websites more backlinks and target keywords on backlinks to get higher rankings in Search engines for particular keywords.
Now, coming to the point about search engines, there are a lot of search engines on the web that people use, but checking my website referral stats, i see the most commonly used search engines are -

Google
Google (Images)
Yahoo!
Windows Live
MSN Search
Ask.com
Baidu
AltaVista
Other search engines
Alexa is also believed to a web information system, but is not that accurate.

Google, Yahoo and recently most popular Live Search all can bring in traffic through web search and also image search. Every search engine has its own algorithm to rate the websites quality, and by that they rank the websites for every particular keyword.
So, building backlinks won’t bring you to the top of search results in every search engine. There are certain tactics to rank differently in different search engines. This is what the search engine optimists do actually.

And the question is, do you try to optimize your site for ranking in every search engine? Or is it for any one?

I don’t care about search engines like Ask, Altavista etc. but rely on the traffic that i get from Google, Yahoo, Live search engines. They can give thousands of visitors a day if we rank for a few top keywords. And i believe most of the people here might be having a go mainly for Google than any other search engines.
I have learnt and read in many blogs that a few things or tricks to optimize your site for good SERP results in both Google and Yahoo are the same. These are like Meta tags, Keyword Targeted URL, Maintaining an average keyword density of about 5-6%, Publishing sitemaps. These all things help for better ranking in both Google and Yahoo, not sure about the Live search though!

The most important search engine for webmasters?
I think it is Google, not because i or a few others target it for our traffic, but because in most of the instances SEO companies compare search results and rankings of websites in Google rather than any other Search engine.
Secondly, most of the SEO contests like the most recent seocontest2008 also decide the winners of the contest from rankings in Google, judging it as the most potential search engine.
And lastly, when people talk something about Search, their discussion starts and ends with Google, and others are not at all cared. May that speaker be a webmaster or a normal internet user. And recently, there was a site SEOMeter which ranks websites basing on their website crawl rate by Google spiders.

So for which Search engine do you optimize your site?