The unpaid entries in a search engine results page that were derived based on their contents’ relevance to the keyword query versus all the other indexed content on the web.
You will commonly see search traffic broken down into organic and paid. Anyone trying to increase search traffic for their business must understand the difference between these: where each set of results are located, the philosophy behind how the rankings for each of them are calculated, etc.
Problems arise when businesspeople, particularly small business owners, buy vague search engine marketing services. A salesperson might say, “get the #1 listing in search engines X, Y and Z” and an unsophisticated buyer might not seek clarification. If the seller is simply buying paid traffic, the buyer is missing out on all the benefits that come from organic traffic, such as access to searchers who don’t click on ads and expenses that don’t increase with each new visitor.
With the introduction of features like Google OneBox & Universal Search, the organic search results are not as simple as ten ordered text listings. Now the free side include things like videos, images and product listings, giving site owners multiple opportunities to get more “organic” exposure.
see also: SEO Tools
Meet The Google OneBox, Plus Box, Direct Answers & The 10-Pack
Search Engine Land (September 28, 2009)