Search engine marketing, or SEM, is a form of Internet marketing that seeks to promote websites by increasing their visibility in search engine result pages (SERPs). According to the Search Engine Marketing Professionals Organization, SEM methods include: search engine optimization (or SEO), paid placement, and paid inclusion.[1] Other sources, including the New York Times, define SEM as the practice of buying paid search listings.[2][3]
From wikipedia....I hope that its gonna clear you out on the difference between SEM and SEO...
To answer the OP?s question?. SEO and SEM are two distinctively different things and both fall under SES.
SES is the strategy of search engine marketing
SEO is search engine optimization
And SEM in most general terms mean management of things like CPC and other ad buys.
Now if your question is out of all this what acronym is most likely to produce the most profits most reliably and most definitely?
Then the answer is SEM
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