SEM (Search Engine Marketing) is paid advertising that is done in order to promote a website in search engines. SEO (Search Engine Optimization) is organic search positioning. SEO and SEM have the main objective of improving visibility in search engines, of course, one and the other have significant differences. SEM is a pay-per-click (PPC) service, but SEO improves the search result without having to pay for it. On the other hand, with SEM these results can be achieved relatively quickly, for SEO it would be necessary to implement a more long-term strategy as well as to have the website perfectly optimized.
We have a post in our blog about that in spanish, it is about the benefits of using SEO and SEM in your marketing strategy.