Black hat SEO is the use of techniques that are unacceptable to search engines to boost a page's position in search results. These techniques are intended to trick search engines into giving pages higher positions in search results,and they have no benefit to site visitors. The goal is to improve search engine result positions no matter what it takes to do it.

Examples:

Overusing keywords in comment tags, alt tags, and meta tagsOverusing keywords in visible text (Keyword stuffing)

Placing keywords in hidden text (hidden from people, that is) by making their font color the same color as the page background Using Doorway or gateway pages that are stuffed with keywords but that only search engines see because people are redirected to the page with the real content

Using Cloaking techniques for displaying different content to search engines than to people by identifying visitors via IP or via other methods Using link farms solely for the purpose of creating more links to target pagesSpamming forums, blogs, and other social media sites with links (search engines might not penalize sites for this, but it's considered unethical)