SEM is the abbreviation for search engine marketing. Search engine marketing (or also: search engine advertising) is a subarea of online marketing. It includes, above all, targeted advertising in search engines in the form of paid advertisements.
A further distinction can be made here between:
Paid Listings
These are paid listings in the search results of a search engine.
Paid Placements
This is paid advertising on other websites, e.g. in the form of linked banners.