The biggest difference between SEO and SEM is organic vs paid.
SEO = Search Engine Optimization
SEM = Search Engine Marketing
SEO = you can enhance SERP results by tweeking chances on your website
SEM = its all about paid marketings, PPC, banner ads, any paid drivers that help bring traffic to your website.
I encourage you to search on this forum and you will find a wealth of insights.
After this post, all of the other explanations were pretty much trying to say what this post said in a different way.
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