Search engine optimization (SEO) is the art and science of getting pages to rank higher in search engines such as Google. Because search is one of the main ways in which people discover content online, ranking higher in search engines can lead to an increase in traffic to a website.
In Google and other search engines, the results page often features paid ads at the top of the page, followed by the regular results or what search marketers call the "organic search results". Traffic that comes via SEO is often referred to as "organic search traffic" to differentiate it from traffic that comes through paid search. Paid search is often referred to as search engine marketing (SEM) or pay-per-click (PPC).