SEO - Search Engine Optimization

SEO is short form for Search Engine Optimization or Search Engine Optimizer. 

Search Engine Optimization (SEO) is a methodology of techniques, tactics, and strategies used to increase the amount of visitors (page views) to a website by obtaining a high-ranking placement in the search results page of a search engine (SERP).

Example: Google, Bing, Yahoo and other search engines.

SEO makes sure that a site is accessible to a search engine and improves the chances that the site will be found by the search engine. Thus, using the right kind of SEO tactics, strategies and techniques is very important to make your website get caught in search engines.

Site ranks in a search is essential for getting more traffic towards your website as it is common practice for Internet users to not click through pages and pages of search results. The higher a website naturally ranks in organic results of a search, the greater the chance that that site will be visited by a user.
SEO is typically a set of "white hat(ethical practice)" best practices that webmasters and Web content producers follow to help them achieve a better ranking in search engine results.

Some experts use "Black hat" techniques to yield great amount of traffic but it is purely unethical and the website could be banned.

Also see "White Hat"         "Black Hat"