Keyword stuffing or keyword spamming was once a popular method of ranking a site for its targeted keyword. This technique often would have the keyword spammed numerous times in the site’s Meta tags or image tags. Now search engines have evolved and will recognize this.
Website scraping is another black hat technique that copies—or scrapes—a site’s full content and places it on another site or sites. These sites would then spam search engines with their copied content. Duplicate content is now a debatable topic, as to whether or not sites take a hit in rankings if they do not have unique web content. These sites may also violate copyright laws.
Google bowling is basically a huge link farm that points at a competing site in order to bring down the ranking of a competitor. Google bowling messes up the external ranking used by Google and penalizes a sites ranking compared it the competitors. This is another sure way to get yourself banned.
Blog spamming is a popular black hat technique that involves a user sending a pre-generated “spam" comment to multiple websites with a link back to their blogs. Blog spamming is still a common problem for legitimate websites. Comment moderation and a free WordPress plugin for privately-owned blogs called Akismet can help combat comment spam. Akismet works similar to your spam box for your email. Akismet runs tests on comments and if it determines the comment is spam, it will be put aside in the database for 15 days, in which you can review your comments in case there was a “false positive."
Doorway pages—or bridge or gateway pages—are web pages that are targeting one keyword or set of keywords, but redirecting users to another page altogether. In the past doorway pages were a common black hat trick, but now search engine spiders track these doorway pages very easily. If the navigation path does not match the redirection path, it can be flagged and eventually penalized or even banned.
Cloaking is another black hat SEO technique that tries to send a web page to the search engine spiders that is different than the website a user actually sees. Cloaking tricks search engines into thinking their content is different than it actually is.