What is the difference between SEO & SEM?

What is the difference between SEO and SEM? I already know a lot about SEO but have not really heard that much about SEM. What does SEM do? Is SEO a part of the whole SEM process?

Think of SEO as the organic side of things. It’s about optimizing your website to rank higher in search results for free. SEM includes SEO, but it also involves paid advertising strategies like Pay-Per-Click (PPC) ads.