Search engine optimization (SEO) is a set of methods aimed at improving the ranking of a website in search engine listings, and could be considered as a subset of search engine marketing (SEM). The term SEO also refers to "search engine optimizers," an industry of consultants who carry out optimization projects on behalf of clients' sites.

Search engines display different kinds of listings in the search engine results pages (SERPs), including: pay per click advertisements, paid inclusion listings, ad banners, and organic search results. SEO is primarily concerned with advancing the goals of a website by improving the number and position of its organic search results for a wide variety of relevant keywords and phrases related to the web site's audience. SEO strategies may increase both the number and quality of visitors and possibly increased revenue. Search engine optimization is sometimes offered as a stand-alone service, or as a part of a larger marketing effort, and can often be very effective when incorporated into the initial development, design and maintenance of a site. Even after the site is published SEOs can recommend changes to the site's meta tags, contents and structure that will get better results within a short period of time. SEO can be an ongoing process and it may need continous tweaking to be effective.

For competitive, high-volume search terms, the cost of pay per click advertising can be substantial. Terms like 'Mortgage' can cost you anywhere from $5 to $15 per click on major search enginges. Ranking well in the organic search results can provide the same targeted traffic at a potentially significant savings. Site owners may choose to optimize their sites for organic search, if the cost of optimization is less than the cost of advertising. The benefit of SEO is continous and on the long run it will benefit the site owners a lot to have their sites optimized by an expert.  However, every website has to be optimized in order to be found on search engine results.

Not all sites have identical goals for search optimization. Some sites seek any and all traffic, and may be optimized to rank highly for common search phrases. A broad search optimization strategy can work for a site that has broad interest, such as a periodical, a directory, or site that displays advertising with a CPM (cost per thousand impressions) revenue model. In contrast, many businesses try to optimize their sites for large numbers of highly specific keywords that indicate readiness to buy. Overly broad search optimization can hinder marketing strategy by generating a large volume of low-quality inquiries that cost money to handle, yet result in little business. Focusing on desirable traffic generates better quality sales leads, resulting in more revenue. Search engine optimization can be very effective when used as part of a smart niche marketing strategy.

There are various aspects for SEO. It involves creating the a) relevant meta tags (the information search engines use to analyse your site and its objectives), b) relevant content that matches the purpose of your site, c) relevant links from other websites in your category and geography.