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. SEO can target contextual search, local search, and industry-specific vertical search engines.
A typical Search Engine Results Page (SERP)

SEO is marketing by understanding how search algorithms work and what human visitors might search for, to help match those visitors with sites offering what they are interested in finding.