You're right in your definition. SEO (search engine friendly design, link building, etc.) is a subset of SEM. SEO is about improving your organic (non-paid) se

You're right in your definition. SEO (search engine friendly design, link building, etc.) is a subset of SEM. SEO is about improving your organic (non-paid) search results. SEM generally refers to a combination of SEO and PPC (pay-per-click) advertising, and other online marketing methods. That said, I think things like the 37 signals post are too easily classified as SEO when it's really just publicity seeking with no real intent on improving the sites rankings.