SEM (Search Engine Marketing) is…
The practice of marketing your website to search engines. The traditional SEM is done through PPC (Pay Per Click) Campaigns to the major search engines like Google, Yahoo & Bing. This is done through the search engines’ revenue-generating ad services, like Google Adwords. Currently, SEM practices have grown to include paid listings, banner ads, directory submissions, SEO (Search Engine Optimization), social media, email marketing, local search and developing online marketing strategies.
SEO and SEM strategies and tactics are so closely related that many use the terms interchangeably. The way I like to explain it is like this.
SEO is the onsite strategy integrated to make your site search-friendly. For example, determining and using keywords in strategic places like title tags and H1s. This is done on your website to rank your site higher than other in SERPs (Search Engine Result Pages) organically.
SEM is all the other off-site things done to either, A – Use paid ads to generate links and traffic to your site, artificially placing you higher than your completion in SERPs, or B, off-site strategies to raise your organic listing.