When Google was created a decade ago, the goal was to present the most relevant pieces of information from the Web to the screens of the end users. The same principles were behind the creation of other search engines today.
Under the rules of these search engines, it is OK to optimize your webpages to rank better on search engine results. Techniques like article optimization. tagging, and others that conform with the rules of search engines are considered to be ethical and non-harmful to the improvement of relevant searching.
However, there are also techniques that we should avoid. Sometimes we just don't notice it, but we practice SEO techniques that are considered unethical and damaging to the goal of relevant search results. These techniques should be avoided at all cost because once the search crawlers find out about your site's optimization misdeeds, it might spell the end for your SEO campaign.
Among the techniques considered to be black-hat by SERPs are using link farms, keyword stuffing, invisible text, and doorway sites, article spinning, and some more. These techniques have the objective of tricking the crawlers and index your webpage higher than it should be.
For your guidance, link farming means joining a group of other websites that hyperlink to all the other members of the group. There is a thin line between this and proper linking, and that is when the links are designed to spam the crawlers instead of actual referral. Meanwhile, keyword stuffing means putting a lost list of keywords inside your page and nothing else. It's just like an endless sea of incoherent keywords.
Meanwhile, invisible text includes the long list of keywords hidden from user view by, for example, printing them in black over a black background. This is just like keyword stuffing, except that it is more deceptive.
Next are doorway pages, which are basically "fake" pages unseen by the regular user. These kinds of pages were created mainly for the crawlers in an effort to raise the search engine rank. Finally, article spinning is the practice of copying articles from other webpages and changing some text so that the new article won't be marked as duplicate content. In this case, it is not only some black-hat SEO technique. Copyrights may also be infringed.
