SEM stands for Search Engine Marketing, covering a wide range of marketing practices and optimizing pages for search engines.
You can say that SEO or Search Engine Optimization is a subset of SEM.
With SEM, you can go into PPC campaigns where you pay per click.
You can also go into content marketing.
Whereas SEO focuses on on-page optimization and backlink attraction, SEM goes into more nuanced and refined activities that can improve your search engines.
Examples of this are press releases or brand outreach and similar things.