These are also known as Spamexing or Spamdexing.
A link farm is a series of websites that were created solely to display a bunch of links to a collection of websites. These links could also constitute a network of websites interlinking with each other.
Google and major search engines consider them illegal because these sites aim to achieve high rankings for websites that haven't earned those rankings through good content and overall quality.
This strategy is highly dangerous; even if your website has great content, your website can still get penalized or banned if you use this technique because the engines figure that you wouldn't try using this shady technique if you had good content.
How to Guard Against it:
Have the SEO professional tell you specifically how he/she will secure incoming links for you. You want to hear an answer that includes targeting specific, pre-existing, and established websites to gain an incoming link from them to you (preferably without linking back to them.)
If the SEO professional says that he/she will build you hundreds to thousands of webpages across different domains that will link to your website, refuse to work with them because this will severely damage your site's rankings.
It's also a good idea to search your domain name in the major search engines from time to time to see what sites are pointing at you. If you see anything out of the ordinary, including sites that have domain names that are long or don't make much sense, as well as sites that just have long lists of links on them, approach your SEO professional about removing your site from these pages and figuring out how your site appeared on these pages in the first place.
These pages are also known as "Gateway Pages, Advertising Pages, Jump Pages," etc.
Doorway pages are a form of landing page that is often not viewable by human visitors, but is designed for search engines. They will often use a redirect script to automatically point the visitor to another page on the site without the human visitor ever seeing the doorway page. This practice is also known as cloaking, which is considered an illegal practice by Google and other search engines.
The only time Google and the other search engines consider a landing page as a legal practice is if the landing page is in the form of an informative, well-written article that human visitors can read and enjoy, without being tricked into clicking or being redirected to the site's main pages.
How to Guard Against It:
Know exactly what kind of pages are being added to your website and make sure to look at most of them. Directly ask your SEO professional whether any of these pages will automatically redirect to your website's main page. If they say yes, we recommend you do not work with that individual or company because they know they are breaking the rules by employing that technique into your webpages.
Even if they say that they use a special javascript or other redirect script that is considered "legal" or acceptable by Google, your website's ranking will pay for it at some point - even if Google doesn't know about that particular trick yet, you know they will find out about it soon enough.
This is also known as "Keyword Spamming."
Keyword stuffing occurs when you load a webpage full of particular keywords, whether in the meta tags, other script tags, or in the content itself, for no credible or informative reason. The keywords are inserted repeatedly into the page just for the benefit of boosting your rankings.
There are several ways to hide keywords from human visitors, but still have them appear visible to search engines. Some will make the text the same color as the background, others will hide keywords in script tags, and others will use CSS to position keywords off the visible portion of a viewer's screen.
Search engines can determine whether a keyword is being used properly or not - if it's not being used properly, they can penalize or ban your site.
How to Guard Against It:
The only way you can determine whether your site has been stuffed with keywords is by viewing the source code of your website (point your browser to your site, click "View," then "Source Code.") If you see the same keywords repeated hundreds of times everywhere, your page is employing keyword stuffing and your site will be banned for it.
Scraper pages are made up of search results or content that is automatically pulled from dozens or hundreds of websites or search engine results. Because the scraper page has no original content, this is considered a form of plagiarism. These types of sites are often used to display Google Adsense Ads or other ads that pay the site owner each time the ads are clicked upon. However, search engines are now very good at banning these types of sites from their results pages and human visitors can also detect these types of sites very easily as well.
They are easy for human visitors to detect because the sites are usually an illogical mess, composed of snippets of other webpage content or search results. The information on these sites does not make sense when read.
How to Guard Against It:
First, you need to ask your SEO professional whether they will employ such unethical techniques. You then need to monitor their work. Make sure you have access to your website's hosting service so you can view all pages that are hosted on your site's domain. Check the pages at random from time to time to make sure they do not contain this or any other illegal content. Get reports of your site's rankings and search the keywords you rank for. Click through to your site from the search engine results and check the landing page's source code and content to make sure there is nothing fishy or inappropriate.

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