Ever heard of Maslow's hierarchy of needs? It's a theory of psychology that prioritizes the most fundamental human needs (like air, water, and physical safety) over more advanced needs (like esteem and social belonging). The theory is that you can't achieve the needs at the top without ensuring the more fundamental needs are met first. Love doesn't matter if you don't have food.
Now let's look at a trickier example—one where the root keyword arguably does a good job describing what we are selling. Say I own an online jewelry store that sells all types of jewelry. To rank highly for the keyword jewelry would probably be at the top of my search engine marketing goals. And yet this would probably not be a profitable keyword that will drive relevant traffic to my site. That is because, from an organic SEO perspective, you are unlikely to rank highly for this term unless you are a huge, highly authoritative site—or lucky enough to be Jewelry.com, knowing that Google rewards keywords that match website addresses.
The key distinction is that SEM is a broader strategy which combines paid search and SEO tactics. So, if you set up a PPC advertisement, but don't take steps to optimize the ad or the website it links to, this is a standard example of paid search. However, if you optimize your site and your search content with solid keywords, then you put money behind it to boost it on targeted audience's search pages, then you're dabbling in SEM.