The primary aim of SEO and SEM is the same, to get ranked on search engines for organic traffic. However, the major difference is SEO is an organic position whereas SEM is a paid position. Actually, SEM, or search engine marketing is a combination of search engine optimization and pay per click advertising campaigns. SEO builds traffic over time, however, SEM gets immediate traffic, SEO is a long term strategy, where as SDEM gives you immediate results. SEO is an ongoing process, where as SEM is one time setup. SEO aims to improve visibility, where as SEM is basically for sales. SEO is free or costs little money, where as SEM is a paid option.