SEM is a broader term than SEO, and is used to encompass different options available to use a search engine’s technology, including paid ads. SEM is often used to describe acts associated with researching, submitting and positioning a website within search engines.  It includes things such as search engine optimization, paid listings and other search-engine related services and functions that will increase exposure and traffic to your Web site. 
In 2007, Google announced a campaign against paid links that transfer PageRank.[30] On June 15, 2009, Google disclosed that they had taken measures to mitigate the effects of PageRank sculpting by use of the nofollow attribute on links. Matt Cutts, a well-known software engineer at Google, announced that Google Bot would no longer treat any nofollow links, in the same way, to prevent SEO service providers from using nofollow for PageRank sculpting.[31] As a result of this change the usage of nofollow led to evaporation of PageRank. In order to avoid the above, SEO engineers developed alternative techniques that replace nofollowed tags with obfuscated JavaScript and thus permit PageRank sculpting. Additionally several solutions have been suggested that include the usage of iframes, Flash and JavaScript.[32]
DisabledGO, an information provider for people with disabilities in the UK and Ireland, hired Agency51 to implement an SEO migration strategy to move DisabledGO from an old platform to a new one. By applying 301 redirects to old URLS, transferring metadata, setting up Google webmaster tools, and creating a new sitemap, Agency 51 was able to successfully transfer DisabledGO to a new platform while keeping their previous SEO power alive. Additionally, they were able to boost visitor numbers by 21% year over year, and the site restructuring allowed DisabledGO to rank higher than competitors. Their case study is available on SingleGrain.com.

While backlinks are still an important factor used by search engines to determine content quality, linkless mentions are given more weight than ever before. This is partially due to the perception is that linkless mentions are more genuine, unlike black hat SEO techniques like paid links. Social media mentions are also playing an increasingly important role in evaluating website quality.
Google's search engine marketing is one of the western world's marketing leaders, while its search engine marketing is its biggest source of profit.[18] Google's search engine providers are clearly ahead of the Yahoo and Bing network. The display of unknown search results is free, while advertisers are willing to pay for each click of the ad in the sponsored search results.
For example, let’s say you were running the website of an online pet store. You might be wise to create one keyword grouping for all your dog-related products, then one for all of your parakeet-related projects, etc. The next step would be to segment each individual group into smaller subgroups (parakeet cages, parakeet toys, parakeet snacks) and then even smaller groups for each type of product (low-fat parakeet snacks, luxury parakeet snacks… you get the idea). Now your pet store can create individual pages optimized for each small keyword group.
The amplification and link building is always going to be difficult in a space where all related sites are potential competitors. Take for example a website focusing on news for a particular sport. Sure, you might get forums links, social media links, and other lower quality 'no-follow' links, but then but no news outlets, sports clubs, and the like would be interested in linking to your content because they have their own sports journalists. Likewise, fan sites, and others would be more inclined to link to well established traditional media, instead of your 20k per month visitor sports site.
Additionally, there are many situations where PPC (a component of SEM) makes more sense than SEO. For example, if you are first launching a site and you want immediate visibility, it is a good idea to create a PPC campaign because it takes less time than SEO, but it would be unwise to strictly work with PPC and not even touch search engine optimization.
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