SEO, or Web positioning, consists of means to position websites on directories and search engines results according to a precise goal.
To position is not to submit — indeed, submissions are nowadays mostly useless — but to adapt one's website so that it can be well understood and well placed by search engines.
Web positioning is a major marketing lever since it applies at the time the Net surfer defines himself as a "target", by submitting a request to the search engine. By contacting prospective customers only after they qualified, SEO lets you direct your efforts at a highly-qualified audience.
To be efficient, web positioning must integrate characteristics from both your website (e.g., technical parameters, marketing positioning, goals) and your competitive environment.
SEO objective may be an increase in visibility and presence; it may also be generating qualified traffic, thus transactions.
Therefore, web positioning is both a marketing tool for sales or customer relations, and a public-relations tool for enhancing your image and increasing your reputation.