As web designers, we are tasked with creating usable websites that are pleasant to view and navigate while still providing a search engine friendly structure which appeals to a "robot" or "spider" rather than a human. This is the concept of Search Engine Optimization, or SEO.
It is easy to confuse Search Engine Optimization with Search Engine Marketing and many times the terms are used interchangeably because they work together so closely. Both are needed for a successful website. It makes no sense to optimize your site and not market it, as well as it does not make sense to market a website that is not optimized for your marketing efforts.
The chart below illustrates this relationship. This graphic was displayed during a presentation to the Hollywood Beach Business Association in August 2007.
A key factor to note is that optimizing your site for search engines is actually part of the website design process. But the optimization can not be completed until you have begun to establish your search engine marketing strategy.
Example: implementing key phrases into your website is part of optimization (SEO) and web design, while choosing the key phrases is part of marketing (SEM).