Search Engine Optimization (SEO) is the practice of improving the ranking of a website on search engines. The higher the ranking, the more likely people are to find the site. SEM, or Search Engine Marketing, is the practice of buying ads on search engines in order to increase visibility for a website. The main difference between SEO and SEM is that SEO is free while SEM requires payment. Another difference is that SEO requires time and effort to achieve results while SEM results are immediate. Finally, SEO targets organic or natural search results while SEM can target both organic and paid results. In conclusion, SEO and SEM are both important tools for increasing visibility online, but they work in different ways.