SEO (Search Engine Optimization) refers to search engine optimization. SEM (Search Engine Marketing) refers to search engine marketing. Achieving a ranking in search engines is solely about getting listed in search results. SEM, on the other hand, represents a marketing strategy for the entire website or even the whole enterprise within the context of network search engines. Of course, SEO is a prerequisite for SEM — if a company's website doesn't even have a ranking in search engines, then there's no point in talking about search engine marketing!