Maybe you find your answer here :

SEO stands for ?search engine optimization.? It is the process of getting traffic from the ?free,? ?organic,? ?editorial? or ?natural? listings on search engines. All major search engines such as Google, Yahoo and Bing have such results, where web pages and other content such as videos or local listings are shown and ranked based on what the search engine considers most relevant to users. Payment isn?t involved, as it is with paid search ads.