Simply put, SEM is Internet marketing that focuses on improving the visibility of your website by ensuring it appears on a search engine results page (SERP). SEM generally refers to paid search or pay-per-click advertising. Those ads that appear at the top or the side of a Google SERP are an example of businesses utilizing Google’s SEM tool, AdWords.
Search Engine Optimization (SEO)
SEM does not refer only to paid search results; SEO (included within the framework of SEM) is also a process of improving a site’s visibility by ensuring appearance on SERPs. The difference is that SEO relies on organic search results as opposed to paid search to achieve high ranking on a results page. These organic tools include making sure your website has good content (keywords) and backlinks (getting others to link back to your webpage) as well as specific HTML clues and architecture built into the site.
The theory is that an average search engine user will more likely click on websites that appear on the first page of results. Additionally, there is a general conception that the top ranked (earliest) results are the most relevant to the user according to the keywords he/she typed into the search box, so your website will get more views if it has high SEO value.
Social Media Optimization (SMO)
SMO refers to the process of using social media to drive traffic to your website. Social media presence does play a role in SEO value, but SMO focuses on using social media as its own tool, independent of search engines. Social media is generally defined as user-generated communications such as social networking sites (Facebook), blogs, microblogs (Twitter), content communities (YouTube) and the like.