In 1998, two graduate students at Stanford University, Larry Page and Sergey Brin, developed "Backrub", a search engine that relied on a mathematical algorithm to rate the prominence of web pages. The number calculated by the algorithm, PageRank, is a function of the quantity and strength of inbound links. PageRank estimates the likelihood that a given page will be reached by a web user who randomly surfs the web, and follows links from one page to another. In effect, this means that some links are stronger than others, as a higher PageRank page is more likely to be reached by the random web surfer.
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.
Additionally, there are many situations where PPC (a component of SEM) makes more sense than SEO. For example, if you are first launching a site and you want immediate visibility, it is a good idea to create a PPC campaign because it takes less time than SEO, but it would be unwise to strictly work with PPC and not even touch search engine optimization.