search engine optimization - search engine marketing

Search Engine Optimization (SEO) and Search Engine Marketing (SEM) are Two different things altogether.

Search Engine Optimization is what is done to your site to ensure that the end users have a friendly easy to use web site with Search engine Friendly(SEF) Menus and SEF URLS that Google, Yahoo, MSN and Ask.com crawl through and find your content.
SEO is the process of optimizing your website with keywords, web page relationships through links, and design so that you rank well in the search engines for the search terms you are targeting. This doesn’t cost anything unless you pay a professional SEO to assist you.
SEO is a more technical process than SEM and involves a deeper understanding of search engine algorithms.