What are the major differences between SEM and SEO marketing services?

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SEO is the act to stay at the top of the search list without doing any payment.

On the other hand, SEM is quite opposite of SEO which is done with the payment.
Both SEO and SEM aims to increase the visibility of the website.SEM is techniques applied to increase the visibility of the website mainly through paid advertisements.This both helps to increase the visibility of the website.
Both SEO and SEM are Concept of Digital Marketing aims to increase the visibility and popularity of the website on search engines. SEO mainly used to increase the number of website visitors by getting the site to appear high on the result on SERP.
SEM is also considered as internet marketing that helps to increase site visibility through organic search engine results and paid advertising. It also includes SEO tactics as several other search marketing method to boost business and ranking of website keyword on SERP.