Search engine optimization (SEO), considered by many to be a subset of search engine marketing, is a term used to describe a process of improving the volume or quality of traffic to a web site from search engines, usually in natural search results.

Many site owners and consultants engaging in SEO attempt to pursue qualified visitors to a site, and the quality of visitor traffic can be measured by how often a visitor using a specific keyword phrase leads to a desired conversion action, such as making a purchase, viewing or downloading a certain page, requesting further information, signing up for a newsletter, or taking some other specific action.

The term SEO can also refer to "Search Engine Optimizers," a term adopted by an industry of consultants who carry out optimization projects on behalf of clients, and by employees of site owners who may perform SEO services in-house.

Google brought a new concept to evaluating web pages. This concept, called PageRank, has been important to the Google algorithm from the start. PageRank is an algorithm that weights a page's importance based upon the quantity and quality of incoming links. PageRank estimates the likelihood that a given page will be reached by a web user who randomly surfs the web, and follows links from one page to another. In effect, this means that some links are more valuable than others, as a higher PageRank page is more likely to be reached by the random surfer.

SEOs widely agree that the signals that influence a page's rankings include:

- Keywords in the title tag.
- Keywords in links pointing to the page.
- Keywords appearing in visible text.
- Link popularity.
- PageRank of the page (for Google).
- Linking from one page to inner pages.
- Placing punch line at the top of page.