Why is it important to you?

The Search Engine market is fragmented, with Google commanding the majority of the market share (over 80%). If there is one search engine that you want to focus on, that would be Google. Microsoft's announced bid for Yahoo! (subjected to regulatory approval) might increase Microsoft's share in the search market, but I still don't think they will be able to challenge Google's dominance anytime in the near future.

In 2007, a survey concluded that over 50% of traffic originated at a search engine. According to a 2007 report published by eMarketer, over 20% of U.S Internet users use a search engine at least 4-5 times in a day. According to Internet Measurement firm, Hitwise, Search Engines were the highest sources of traffic during the peak Christmas shopping season, representing 57% of upstream traffic. Google alone was responsible for 18% of the upstream traffic, followed by Yahoo with 6%.

The above figure highlights the importance of SEO as a potential marketing tool in attracting visitors to your site.