SEM stands for search engine marketing - the process of gaining traffic from, or visibility on, search engines. The phrase is also sometimes shortened to “search marketing".
SEM is often used to describe acts associated with researching, submitting and positioning a website within search engines to achieve maximum exposure of your website.
SEM is an umbrella term that covers two broad areas:
Gaining traffic through free SEO efforts
Gaining traffic through paid search advertising
SMO is one type of SEO tactic - a set of methods for generating publicity through social community, social media, online communities and social networking websites (including blogging, blog commenting and RSS feeds)..