Search Engine Marketing is a marketing technique that increases visibility of the website on Search Engine Result Pages[SERPs].
SEM includes both SEO and Paid marketing. Many SEO company provide the services to increases the visibility of website on the Organic search result and those website owner pay the fees for this SEO service.
Paid Marketing is a process to show the website on Search result pages for paying fees to Search Engine.
[SEM = SEO + PPC]
Comparison between SEO & SEM :
SEO is one of the branch of SEM or SEO is subbranch of SEM. Both works for increasing the visibility of the website.
SEO work for getting improvement on Page Rank of the website and improve the ranking of the search result.
SEM involves other method to getting more search engine visitors using PPC advertising.
SEM & SEO both involve to increase the visibility of the website in Search Engine Result pages.
In SEO , website need to be optimize the webpages.
SEO work on Organic Search Result that is the normal Search Engine Search result.