Black hat techniques are sometimes referred to as search engine spamming. One example is repeating an important keyphrase many times on the homepage. Another is using text that it is the same colour as the background of the page, so the keyphrase is visible to the search robots and algorithm, but not to the human reader. Engineering pages for robots is a practice known as ‘cloaking’. Other examples of spamming which are well known are doorway pages and link-farms.
The use of unaccepted or frowned upon SEO practices in order to get higher rankings and more traffic. These websites are usually caught and removed from the search engines.
To put it a simple way, dirty techniques that Google doesn't like. And a lot of them, if caught, can lead to being blacklisted from Google. Which is basically a death sentence for SEO and websites.
a lot of services would be "i can get you 100k+ links instantly" or "i can get you 10,000 high PR blog comments in 1 day" ultimately spamming your site around the web which google will find out about as there is no organic way of making 10,000 blog posts in one day
Black Hat" techniques are unethical methods that some Web site owners use to get their site listed on search engines. Here's a list of three common "Black Hat" techniques you should avoid since they could get your site banned. Keyword stuffing: Keyword stuffing is the overuse of keywords in a piece of content. Generally, this is repeating the same keywords over and over just to achieve a higher search engine ranking. Invisible text: Quite a few cheap sites use this tactic, which involves making keywords to be the same color as the background, so readers can't see the massive amounts of keywords, but search engine spiders can. Doorway pages: These are pages that regular visitors cannot see, but search engine spiders can. They are used to trick the search engines so that the site gets a higher ranking. Not only do they go against the rules, but they also hurt the visitor's experience.