Sounds like a very simple concept but many traditional marketers and CMO’s still think that SEM refers to buying keywords such as Adwords.
SEM (search engine marketing) encompasses both SEO and PPC.
SEO stands for search engine optimization; this is also referred to as organic search or natural search.
So in short, next time someone mentions SEM, remember that it’s both SEO and PPC.
SEM is Not Just PPC « Tourism Online Marketing – Tips & Online Strategies