SEM marketing can be defined as a set of ideas, tools, strategies, and technicalities that optimize the visibility of websites through search engines. The main idea is to position themselves among the results of Google and other search engines at the exact moment users search for specific keywords.
Google search pages display two types of results: organic and natural. To display them, an algorithm shows which websites best respond to a specific query. Google's algorithm is based on the website's content and links from other pages.
Paid results are displayed in the column at the top right. To do this, the advertiser pays a certain amount for each click they receive. To obtain traffic, advertising systems such as Google Ads will be necessary. This system is also called pay-per-click (PPC) or cut-per-click (CPC).
In practice, SEM refers to search engine advertising, that is, paid results. It's better to use SEM as a synonym for "search engine advertising."