Search engine optimization (SEO) is a set of techniques that tailors your web pages to place well on search engines. Each uses different criteria to determine how to rank web pages. Optimization formats the content of a web page so that it best meets the criteria of what a search engine wants to "see" from a page for top rankings. This can include semantic markup, carefully selected and placed , coming to and from the page, and literally dozens of other factors that the search engine may use to determine the value and credibility of your web site. There are endless ways to manipulate a page to affect search engine ranking, and some of them are considered unethical or unfair. Search engines are constantly changing their criteria if a new method is discovered that exploits a search engine unfairly.