Search engine marketing (SEM) is a form of Internet marketing that involves the promotion of websites by increasing their perceptibility in search engine results pages (SERPs) primarily through paid advertising.
SEO is an abbreviation for Search Engine Optimization, which is the art of having your website optimized, or attractive, to the major search engines for optimal indexing.
Social media optimization is all about exposing your website on social media channels like Facebook twitter; it helps you in building good social media reputation with more traffic, which is considered by Google while ranking your website in search results.