SEO is used to optimize your site rankings in search engines whereas SMO involves working on social media sites and creating a viral marketing buzz to your business.
SEO is the process of improving the quality of our website's traffic via search engines, this can be done by effective On-page and Off-page optimization. Whereas, SMO is process of obtaining the traffic via social media sites, this can done by placing FB "Like" button/"Tweet" share button in your website in-order to share your content in those websites.
In Search Engine Optimization you have to optimized the website according to search engine guidelines while in Social Media Optimization you have to market your site through social media..