Organic search / search engine optimization (SEO) is the traditional form of Search Engine Marketing (SEM). Organic search is a technique of refining your web presence to maximize rankings in a search engine (one that uses an algorithm -- as opposed to a financial bidding system -- for ranking web pages) and drive customers to your web site.
Some factors search engines take into account when ranking results include: keywords, content, site design, page load speed, inbound links, and link "quality".
Organic search is a relatively inexpensive option in that payment is not provided to a search engine for ranking. While payment is not made to a search engine directly, many companies choose to outsource their organic search campaign to a company such as KeywordFreelancer which has professionals on board who stay on top of the latest industry changes.