SEO it's the process of getting traffic from the “free,” “organic,” “editorial” or “natural” search results on search engines. All major search engines such as Google, Bing and Yahoo have primary search results, where web pages and other content such as videos or local listings are shown and ranked based on what the search engine considers most relevant to users. Payment isn’t involved, as it is with paid search ads.
SEM (Search Engine Marketing) is the process of gaining website traffic by purchasing ads on search engines. “Search Engine Marketing” was once was used as an umbrella term to encompass both SEO (search engine optimization) and paid search activities. Over time, the industry has adopted the SEM acronym to refer solely to paid search.