SEM search placements include an “Ad” designation. SEO does not. Search results that appear as a result of SEM or SEO look different on SERPs. Paid ads that receive placement through SEM tactics are often identified as an ad (e.g., by an icon appearing next to the placement), whereas the search results that appear as a result of organic SEO are not marked in such manner.
Description meta tags are important because Google might use them as snippets for your pages. Note that we say "might" because Google may choose to use a relevant section of your page's visible text if it does a good job of matching up with a user's query. Adding description meta tags to each of your pages is always a good practice in case Google cannot find a good selection of text to use in the snippet. The Webmaster Central Blog has informative posts on improving snippets with better description meta tags18 and better snippets for your users19. We also have a handy Help Center article on how to create good titles and snippets20.
Google is falling into a familiar pattern. First, they offer web publishers increased visibility and SERP display options. Next, they incent participation in specific formats and data structures. Finally, they take that data for themselves, changing the SERPs to favor advertising, their own properties, and/or instant answers that can reduce publisher traffic. For web marketers, it's a prisoner's dilemma. In this presentation, Rand will show data on how Google is being used today, how it's changing, then dive into strategic initiatives and specific examples of how savvy players can build a moat to protect against long-term risk.

SEM results show to a select target audience. SEO results do not. While successful SEO and SEM strategies are driven by a plan to connect with a select audience, you can only specify that target audience through SEM. Through SEM, you can (depending on the publisher) select what audiences you want to see the search results by assigning filters based on age, location, income, habits, and more. Through SEO, you cannot specifically choose who will see your search results.
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Brian, I have a burning question regarding keyword placement and frequency. You wrote: “Use the key in the first 100 words … “. What else? I use Yoast and a WDF*IDF semantic analysis tool to check the content of the top10 rankings. Pretty often I have the feeling I overdo it, although Yoast and WDF/IDF told me I use the focus keyword not often enough.

Brand new keywords sound super tricky to find — except for a ton of easy ones that come around every January: simply adding the year to whatever keyword you’re targeting. People can start getting traffic from “2020” keywords long before they show up with any kind of search volume in typical keyword-research tools, since their data lags. (Hat tip to Glen Allsopp, who I got that idea from.)
This relationship between rankings and clicks (and traffic) is strongest amongst the top 3 search results. However, changing layout of the search results pages is constantly changing, with the inclusion of Google’s Knowledge Graph data and the integration of Universal Search elements (SERP Features) like videos, maps and Google Shopping ads. These developments can mean that the top 3 organic rankings are no longer the 3 best positions on the SERP. This has been demonstrated in heatmap and eye-tracking tests.

Companies that employ overly aggressive techniques can get their client websites banned from the search results. In 2005, the Wall Street Journal reported on a company, Traffic Power, which allegedly used high-risk techniques and failed to disclose those risks to its clients.[15] Wired magazine reported that the same company sued blogger and SEO Aaron Wall for writing about the ban.[16] Google's Matt Cutts later confirmed that Google did in fact ban Traffic Power and some of its clients.[17]