Search engine Optimization means optimizing your web page in order to improve the volume or quality of traffic to a web site from search engines via "natural" or un-paid ("organic" or "algorithmic") search results as opposed to search engine marketing (SEM) which deals with paid inclusion

Search engine marketing is a form of Internet marketing that seeks to promote websites by increasing their visibility in search engine result pages (SERPs) through the use of paid placement, contextual advertising, and paid inclusion.

And there is another term which is called Social Media Marketing, It is a term that describes use of social networks, online communities, blogs, wikis or any other online collaborative media for marketing, sales, public relations and customer service. Common social media marketing tools include Twitter, blogs, LinkedIn, Facebook, Flickr and YouTube.

Hope this helps

Regards
Smita..