Search engine optimization (SEO), a subset of search engine marketing, is the process of improving the volume and quality of traffic to a web site from search engines via "natural" ("organic" or "algorithmic") search results. Usually, the earlier a site is presented in the search results, or the higher it "ranks," the more searchers will visit that site. SEO can also target image search, local search, and industry-specific vertical search engines. In Internet marketing, search engine marketing (SEM), is a set of marketing methods to increase the visibility of a website in search engine results pages (SERPs). SEM strategies include: Search engine optimization attempts to improve rankings for relevant keywords in search results by improving a web site's structure, content, and relevant backlink count. Pay per click advertising uses sponsored search engine listings to drive traffic to a web site. The advertiser bids for search terms, and the search engine ranks ads based on a competitive auction as well as other factors. Paid inclusion feeds listings into search engines, typically comparative shopping sites like Nextag. Social media optimization promotes by placing ideas within online communities with the hope that they will spread virally. Video Search Marketing attempts to promote a business by strategically placing short video clips on websites such as YouTube to be picked up search engine spiders or promoted through internal rating systems. A typical Search Engine Results Page (SERP) As a marketing strategy, SEO considers how search algorithms work and what people search for in order to increase a site's relevancy. SEO efforts may involve a site's coding, presentation, and structure, as well as fixing problems that could prevent search engine indexing programs from fully spidering a site. Other, more noticeable efforts may include adding unique content to a site, and making sure that the content is easily indexed by search engines and also appeals to human visitors.