Organic search is a method for entering one or several search terms as a single string of text into a search engine. Organic search results, appear as paginated lists, are based on relevance to the search terms; and exclude advertisements. Whereas, non-organic search results do not filter out pay per click advertising.
The Google, Yahoo!, and Bing search engines insert advertising on their search results pages. The ads are designed to look similar to the search results, though different enough for readers to distinguish between ads and actual results. This is done with various differences in background, text, link colors, and/or placement on the page. However, the appearance of ads on all major search engines is so similar to genuine search results that a majority of search engine users cannot effectively distinguish between the two.
Because so few ordinary users (38% according to Pew Research Center) realized that many of the highest placed "results" on search engine results pages (SERPs) were ads, the search engine optimization industry began to distinguish between ads and natural results. The perspective among general users was that all results were, in fact, "results." So the qualifier "organic" was invented to distinguish non-ad search results from ads.
The term was first used by Internet theorist John Kilroy in a 2004 article on paid search marketing. Because the distinction is important (and because the word "organic" has many metaphorical uses) the term is now in widespread use within the search engine optimization and web marketing industry. As of July 2009, "organic search" is now common currency outside the specialist web marketing industry, even used frequently by Google (throughout the Google Analytics site, for instance).
Google claims their users click (organic) search results more often than ads, essentially rebutting the research cited above. A 2012 Google study found that 81% of ad impressions and 66% of ad clicks happen when there is no associated organic search result on the first page. Research has shown that searchers may have a bias against ads, unless the ads are relevant to the searcher's need or intent 
The same report and others going back to 1997 by Pew show that users avoid clicking "results" they know to be ads.
Incidentally, according to a June 2013 study by Chitika, 9 out of 10 searchers don't go beyond Google's first page of organic search results, a claim often cited by the search engine optimization (SEO) industry to justify optimizing websites for organic search. Organic SEO describes the use of certain strategies or tools to elevate a website's content in the "free" search results.
Organic SEO refers to the method that all the procedures for search engine optimization are followed by the proper method of taking position on the search result
- May/June 2004 Tracking Survey Pew Internet and American Life Project
- "New research: Organic search results and their impact on search ads". Google. Retrieved 6 August 2012.
- Jansen, B. J. and Resnick, M. 2006. An examination of searcher's perceptions of non-sponsored and sponsored links during eCommerce Web searching. Journal of the American Society for Information Science and Technology. 57(14), 1949-1961.