Wikipedia - SEO
Search engine optimization is the process of editing and organizing the content on a webpage or across a website to increase its potential relevance to specific keywords on specific search engines and importantly ensuring that external links to the site are correctly titled and in abundance. This is done with the aim of achieving a higher organic search listing and thus increasing the volume of targeted traffic from search engines.

SEO is one of the key Web Marketing activities and can target different kinds of searches, including image search, local search, and industry-specific vertical search engines.

SEO considers how search engines work and what people search for. Optimizing a website primarily involves editing its content and HTML coding to both increase its relevance to specific keywords and to remove barriers to the indexing activities of search engines. Sometimes a site's structure (the relationships between its content) must be altered too. Because of this it is, from a client's perspective, always better to incorporate Search Engine Optimization when a website is being developed than to try and retroactively apply it.
From Wikipedia, the free encyclopedia