Search engine marketing (SEM) is related to SEO in that they both deal with digital marketing outreach on search engines like Google and Bing. However, SEO typically refers to internal, organic website optimization while search engine marketing is commonly known as advertising through a paid media budget. While SEO will help your website traffic increase through natural algorithmic means, SEM earns traffic through the process of purchasing ads on search engines. In the image to the left, you will see a search engine results page that is the result of typing the keyword “digital marketing” into Google. The top four results have a small green box that says “ad” underneath the primary title link. This signifies paid advertising, or search engine marketing, where a brand or business has paid Google to display their ads at the top of the page for that specific keyword.
The SEO starter guide describes much of what your SEO will do for you. Although you don't need to know this guide well yourself if you're hiring a professional to do the work for you, it is useful to be familiar with these techniques, so that you can be aware if an SEO wants to use a technique that is not recommended or, worse, strongly discouraged.