Organic search results are the list of Web sites that "naturally" come up when a user searches specific keywords (as opposed to paid listings that search engines sell).
Organic listings appear solely because a search engine has deemed it editorially important for them to be included, regardless of payment. Note: Some search engines include paid listings at the top of the organic listings even though these "sponsored links" are paid for.
In a 2004 iProspect report, it states that users click on organic search results more often than they do paid listings:
Google - 72.3% organic, 27.7% Paid ads
All engines - 60.5% organic, 39.5% paid ads