Keyword Stuffing: The search engines study the keywords included in a webpage to index the webpage or a site. So, some people increase the keyword density in their web pages to get a higher ranking. Keywords should be 2 to 4 % of the total world count, increasing keyword density beyond this is known as black hat SEO as it is against the SEO guidelines of Google.
Cloaking: In this technique, the web pages are coded are in such a way that visitors see different content in a webpage and search engines see different content on the same webpage. Increasing ranking in this way is also against the guidelines of search engine.
Doorway pages: These pages are rich in keywords and do not have quality content and relevant information. They redirect users to a different page to increase the ranking of that page. It is also against the guidelines of Google.
Hidden Text: It is the text that can be viewed by the search engine, but visitors cannot view. It is used to include irrelevant keywords and hide text or links to increase the keyword density and improve the internal link structure.
Article Spinning: It refers to rewriting an article multiple times to produce its different copies in such a way that each copy looks different from other copies and is treated as a new article.
Duplicate Content: It is the content that is copied from one site and uploaded to another site. It is known as plagiarism.