SEM and SEO

I think this was a good way to put it...

SEM (Search Engine Marketing) is the broad category of tools that a company will utilize in order to bring more traffic to a website or more attention to a product. It increases the “magnitude” of a site, attracting as many searches as possible.

SEO (Search Engine Optimization) is more specifically the tool of increasing the search-ability of a website (or optimizing) so that it can more readily be found in all search engine listings, including organic and natural listings.

So SEM includes SEO and a lot more as well. SEO is like a sub category of SEM

Search engine marketing, or SEM, is a form of Internet marketing that seeks to promote websites by increasing their visibility in search engine result pages (SERPs) through the use of paid placement, contextual advertising, and paid inclusion.

SEM - The act of marketing a web site via search engines, whether this be improving rank in organic listings (search engine optimization), purchasing paid listings (PPC management) or a combination of these and other search engine-related activities (i.e. affiliate programs, shopping feeds or link development).

SEO - The act of altering a web site so that it does well in the organic, crawler-based listings of search engines. In the past, has also been used as a term for any type of search engine marketing activity, though now the term search engine marketing is more commonly used as an umbrella term.