Search engine marketing, (SEM), is a form of Internet marketing that seeks to promote websites by increasing their visibility in search engine result pages through the use of, paid placement, contextual advertising, and paid inclusion, this includes PPC, Google Adwords, Banner Ads etc
Search engine optimization is "optimizing" website pages to achieve higher ranking in search results via specific keyword searches associated to the website, SEM is utilizing various means of "marketing" a website in order for it to become more relevant in regard to search engine searches and their rankings.
SEM is NOT SEO and vice versa. SEM constitutes Ad words;; which comprises pay per click, article submissions, advertising and making sure SEO has been done.
By analysing your key words and web site structure SEM can be used to maximum benefit, through better configuration of campaigns to reflect the nature of the product, advert and landing pages.
SEM is a constant and tedious task. It frequently needs to be updated and monitored continually.
Another part of SEM is Social Media Marketing (SMM). SMM is a type of marketing that involves exploiting social media to influence consumers that one company's products and/or services are valuable.
You can do SEM without doing SMM but you can't do SMM without doing SEM because SMM is a higher level of SEM.