SEO ( Search Engine Optimisation ) is the active practice of optimizing a web site by improving internal and external aspects in order to increase the traffic the site receives from internet users in either geo targeted or general users.

As an Internet marketing strategy, SEO considers how search engines see the web site data and what internet users search for. Optimizing a website primarily involves editing its content and HTML and associated coding commonly known as on page SEO to both increase its relevance to specific keywords and to remove barriers to the indexing activities of search engines which mainly search the relevent contents of the respective web sites.

The optimisation of web sites for search engines started in the mid-1990s.The popularity of a search engine is determined by its ability to produce the most relevant results to any given search.

The white and Black

SEO Has different catagories like White hat SEO and Black hat SEO .
White hat are those website designers that "play nice" and try to follow all of the search engine guidelines to optimize their site and try to distribute their website content in a guided manner.
Another class of techniques, known as black hat SEO or spamdexing, uses methods such as link farms, keyword stuffing and article spinning that degrade both the relevance of search results and the user-experience of search engines. Search engines look for sites that employ these techniques in order to remove them from their indices.