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. Other sources, including the New York Times, define SEM as the practice of buying paid search listings.

Search engine marketing involves click costs. Search engine optimization works through free traffic. Those two facts are the basis of a popular myth: that it's easier to get good ROI through SEO than it is to get the same ROI through SEM.

In SEM, you decide the landing page your visitors see. In SEO, a search engine spider decides on the landing page visitors see. That's a difference in control, and that difference makes all the difference.

In both SEO and SEM, you only hit ROI if your searchers convert. An SEM firm might require a higher initial investment than an SEO firm does (because SEM requires the management cost, plus the PPC cost; while SEO only costs the price of management alone) - but you're also likely to get better conversions through SEM, because you're likely to get better landing pages through SEM. And it's only conversions that will get you ROI.

So good SEO is absolutely vital to strong internet marketing. But saying that SEO will get better ROI than SEM - because organic traffic is free - ignores the most important element of SEM: control. And it's control that makes all the difference between getting traffic, and getting traffic that converts (http://www.dmnews.com/SEO-Isnt-SEM/article/89604/).