What is SEO?

Search engine optimization (SEO) is the process of improving the volume and quality of traffic to a web site from search engines via "natural" ("organic" or "algorithmic") search results for targeted keywords. Usually, the earlier a site is presented in the search results or the higher it "ranks", the more searchers will visit that site. SEO can also target different kinds of searches, including image search, local search, and industry-specific vertical search engines.

As a marketing strategy for increasing a site's relevance, SEO considers how search algorithms work and what people search for. SEO efforts may involve a site's coding, presentation, and structure, as well as fixing problems that could prevent search engine indexing programs from fully spidering a site. Another class of techniques, known as black hat SEO or spamdexing, use methods such as link farms and keyword stuffing that tend to harm search engine user experience. Search engines look for sites that employ these techniques and may remove them from their indices.

The initialism "SEO" can also refer to "search engine optimizers", terms adopted by an industry of consultants who carry out optimization projects on behalf of clients, and by employees who perform SEO services in-house. Search engine optimizers may offer SEO as a stand-alone service or as a part of a broader marketing campaign. Because effective SEO may require changes to the HTML source code of a site, SEO tactics may be incorporated into web site development and design. The term "search engine friendly" may be used to describe web site designs, menus, content management systems, URLs, and shopping carts that are easy to optimize.

Also,
Search Engine Optimization (SEO) is anything that improves the internal and external aspects of a website in order to increase the traffic the site receives from search engines.

The majority of web traffic is driven by the major commercial search engines Google, Yahoo and MSN. If your site cannot be found by these search engines or your content cannot be put into their databases then you might just as well not exist.

Be careful because 99% of the SEO advice you can read on the web is recycled crap. There are only two reliable and up to date professional tutorials on the subject that I know of http://www.seomoz.org/article/beginners-... and http://www.seo-blog.com/tutorial.php

Read those and you can't go wrong!