SEO (Search Engine Optimization) and SEM (Search Engine Marketing) are key strategies for online visibility but serve different purposes. SEO focuses on optimizing your website to rank higher in organic search results, improving visibility through techniques like keyword optimization, backlink building, and on-page improvements. SEM, on the other hand, involves paid advertising to appear in search results, such as through Google Ads. SEM provides immediate visibility but requires ongoing investment, while SEO aims for long-term, cost-effective results.