The primary difference between SEO and SEM lies in that SEO fuels organic search traffic while SEM drives traffic and visibility from pay-per-click (PPC) advertising. Some aspects of SEO include developing quality content, enhancing user experience and building links. On the other hand, SEM revolves around targeted ad campaigns and applying key performance indicators such as click-through-rates (CTR) and cost-per-click (CPC) for advertising campaigns. Ultimately, despite their differences, they achieve optimal and effective digital marketing results when implemented together.