Though all these terms are related to the search engine and the optimization of websites and other accounts, there is slight difference which makes them totally different from each other. These terms are explained below:

SEO: SEO stands for search engine optimization. It is a process of optimizing the Online ranking of any website or web pages upon the search engine as the result of natural, organic or earned results which are displayed upon the search engine results page. For the purpose, you need to work upon the On Pagecontentand other techniques like on-page and off-page SEO.

SEM: SEM stands for the search engine marketing which is a process of affecting or increasing the search engine ranking of any website or webpage with the help ofpaid advertisement, such as, with the help of Google Adwords.

SMO: SMO stands for social media optimization, which is a process of application and usage of different outlets and mechanisms to affect and increase of the publicity and awareness of social media page with products, services or event exhibited upon the page. 

SMM: SMM stands for the social media marketing. This is the process of working upon social media channels to affect the presence of social media pages of brand or business to promote any product line or services.