Search engine optimization (SEO) is the process of improving the visibility of a website or a web page in search engines via the "natural" or un-paid ("organic" or "algorithmic") search results.
As an Internet marketing strategy, SEO considers how search engines work and what people search for. Optimizing a website may involve editing its content and HTML and associated coding to both increase its relevance to specific keywords and to remove barriers to the indexing activities of search engines.
Source: http://en.wikipedia.org
Article source: http://en.wikipedia.org/wiki/Search_engine_optimization