SEO (Search Engine Optimization) and SEM (Search Engine Marketing) are both strategies used to increase visibility in search engine results but differ in their approaches and outcomes. SEO focuses on optimizing a website to enhance its rankings in the organic search results. It involves improving website content, enhancing the user experience, and building backlinks. SEM, on the other hand, includes SEO tactics but primarily refers to paid advertising strategies, such as PPC (pay-per-click) advertising. SEM aims to increase visibility more quickly by placing ads in search results for specific keywords, and it typically yields immediate results.