Search Engine Optimisation refers to process of developing your website to increase it’s visibility on search engines for search queries relevant to your business. SEO affects the natural or organic listings (sometimes called rankings) in the search engine results pages.
It is generally accepted that organic listings attract 65-70% of the clicks for a given results page. Unlike paid for listings (PPC), you are not charged when searchers click through to your site, so the business benefits of ranking well in the organic listings are clear.