Simple Differences Between SEO and SEM
Search Engine Optimization (SEO) is the process of optimizing the web site for the purpose of getting free traffic from search engines. An optimized web site is more easily understood by Search engine crawlers, so there are more chances of ranking higher in Search Engines Result Page (SERP).
Search Engine Marketing (SEM) is the process of getting more visibility in search engines by paid listing. In SEM, you can see the paid ads on the Top, Left side or Bottom of the Search Engines Result Page (SERP).