Unlike White Hat SEO, Black Hat Search Engine Optimization techniques should be avoided at all costs. One of the most common of these is hiding content on a webpage, which means that only search engines ignore the content, while people who visit the site see it normally. Nowadays, however, search engines notice this and can lead to penalties or even removal of the site from search engine results.
Another common Black Hat technique is keyword bulking, which usually means filling in too many keywords with web page metadata, a practice also followed by Gray Hat SEO experts. This too is effectively detected by the search engines and can be subject to sanctions.
Link farming, or the (insane) addition of links to websites to other websites, was once a heaven for many SEOs, but this has also been fixed. Nowadays, buying or adding links to unrelated pages does not bring in meaningful traffic and can, at worst, lead to the removal of the site from the search engine results.