What's the ddifference between SEO & SEM?

Many people confuse these two terms and use them interchangeably but there is a difference between SEO and SEM. Search engine optimization (SEO) is a component of search engine marketing. It is the act of optimizing a website for organic or natural search engine listings. Search engine optimization is one of the most cost-effective functions of a search engine marketing campaign, but it is only one element of SEM.

Search engine marketing is the act of marketing a website via search engines, whether by improving rank in organic or natural listings, paid or sponsored listings or a combination of both.
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SEO is short of Search Engine Optimization and SEM is short of a Search Engine Marketing. Both are used to increase visibility of site on search engine. SEO is one marketing technique of SEM, it has a different tools like Social bookmarking, Forum posting, Blog commenting, Article submission to increase traffic on site. SEM includes the PPC and other search engines marketing techniques.

SEO - is the optimization of a certain website in the search engine world. it is the method of helping a website ranks in the SERP (search engine result page)

SEM - primarily focuses on internet marketing, uses the search engine for marketing sales and strategies.