what is main difference between SEO and SEM?

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SEM holds the benefit in the fact that you can get instant qualified traffic to your web page. How much traffic is only dependent on your budget. So what are the cons of SEM? Firstly, if you do not properly optimize your ads, as in Google's case, you run the risk of burning through your marketing budget very fast and having a low conversion rate to show for it.
SEO on the other hand is a longer term approach, and utilizes many best practice web design standards. Good original content, well implemented on page optimization and a solid link building campaign is usually more than sufficient for SEO. Apart from achieving good rankings, you are also developing a solid site that your visitors like to visit and stay on.