Simply put, SEO (Search Engine Optimization) refers to the use of organic methods to appear in search results, while SEM (Search Engine Marketing) refers to the use of paid methods to appear in search results.
Both methods improve visibility and increase traffic to your website: whetther through an SEO strategy (such as content marketing) or through paid advertisements, the truth is that either of these strategies end up increasing your website's visibility, although in the case of SEM the results are more immediate.

So how do these two concepts differ?
As mentioned above, one of the main differences between SEM and SEO is that, for the first case, the results are more immediate, that is, when launching for example a Google ads campaign, it is easily visible the increase in the reach and number os clicks to your website. SEO can take time to produce results.
However, it is important to note that although SEM produces faster results, SEO produces longer lasting results, this is because website optimization makes the results improve over time, while once a paid campaign ends, the visibility also ends.
Another big difference between these two concepts is, obviously, in PPC (pay per click), since in the case of SEM there is always a cost associated with each click, while in SEO the clicks are free.

Although these are different channels, they should not be seen as mutually exclusive.
Once you understand the purpose of each of them, the goal should be to analyze how these two methods together can contribute to the growth of your business.