Both SEO and SEM help your brand appear in search results and drive more traffic to your website. The main difference between these two is that SEO is an organic strategy, while SEM is a paid strategy.
Organic SEO is less costly, but it might take a longer time to show results. SEO is something that you acquire over time and typically over a long time. Nevertheless, it gives you search credibility that you might not get with SEM. On the other hand, several circumstances make more sense to incorporate SEM as part of your marketing strategies. For example, if your business is new and has little to no online presence, it is good to create a PPC campaign for immediate visibility. Once your business and website are established and already have some online authority, it is wiser to consider SEO as your long-run marketing strategy.