SEO is always better!!!! but SEM is also growing and need hand-to hand support with SEO....

SEO is a subset of SEM.

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As already mentioned above in the post....SEO is a subset of SEM
SEO= Search Engine Optimization
SEM= Search Engine Marketing

SEM is the marketing using Google Search Engine, by virtue of which your website ranks in the organic search of the Google and get the visits and end up with a sale of your product !!

But

SEO is the technology of how to rank your website in Google Search engine Ranking.

I hope this ll help you !!

Nuke

Yes, SEO comes under SEM..

SEM = search engine marketing
SEO = search engine optimization