What is the different between search engine marketing and search engine optimization?

Quite simply, Search Engine Marketing (SEM) is the process of advertising your website through other websites in an effort to increase you search engine rankings and well as overall exposure. Search Engine Optimization (SEO) is the process of ensuring your website is built for maximum effectiveness and follow all the current search engine indexing technology. SEM is now acknowledged to be the most important factor that will effect your website in regards to ranking in search engines, however, SEO while not seen as important as it once was, is still regarded as a major factor is sustaining that position that was achieved with SEM.