SEO is subset of SEM. if you are doing SEO with paid advertising it come under the SEM ,SEO make the site better for both search engines and users so before starting paid advertising its need to have done SEO first .
Search engine optimization (SEO) is the process of improving the visibility of a website or a web page in search engines via the "natural" or un-paid. Search engine marketing, (SEM), is a form of Internet marketing that seeks to promote websites by increasing their visibility in search engine.
SEO = Search Engine Optimization = Optimizing your website to rank for specific keyswords or keyphrases in search engines. SEM = Search Engine Marketing = The form of advertising on search engines by using PPC (pay per click) campaigns. Both are forms of having your site appear in the search engine ranking pages (SERPS)..However, one costs your for each click you get and the other does not.
SEO comes under SEM which is search engine marketing. PPC is also coming under SEM. dad is SEM and it has 2 kids - SEO & PPC
Who said SEO is free? Though Google offers is as free, it needs money, resource and effort to bring your website in organic search to drive traffic.
I would just like to add these: SEO - Use on-page optimization factors e.g. keywords & content; - Interlinking - inbound/outbound links; SEM - PPC ads; - Social Media Marketing;
SEO and SEM both are same but 1 similarity is in sem using PPC and affiliate marketing factors so this type is paid in my opinion this is the difference.
Couldn't have said it better myself. Except to include that SEO is Organic and achieved via on-site and off-site methodologies...and SEM is paid marketing.
SEO is a process which involves solutions for a site to appear high in serps, whereas SEM is a process where you market only to get leads/sales.