Most search engines offer two types of search results to their customers: paid results (typically at the top or on the side) and organic or natural results. While paid results can get your Web site to rank higher for a specific keyword phrase (because you've paid to be high in the results), most customers consider these results to be little better than advertising, and will often skip over them in favor of the natural or organic search results.
An organic search is a search that generates results that were not paid advertisements. Many customers feel that these results are a more accurate reflection of what they might want when searching for a specific phrase, because they are generated by popularity and common usage.
When you do SEO or search engine optmization you are attempting to adjust the content of your Web pages to rank well in the organic search results.