Black Hat (or Blackhat) SEO has become the name for the collection of methods that artificially boost web pages' search engines rankings. As a rule, it concerns methods that break the TOS* of most major search engines. Typical black hat techniques include cloaking, keyword and link stuffing, using doorway pages and site hijacking.
Note that most black hat SEO practitioners are not hackers and are not breaking any laws. That said, since they break the search engines' rules, they run the risk of getting their sites banned from the serps.
The term was inspired by old cowboy films, which always had a hero wearing a white hat, and a gang of bad guys with black hats.
* see for example Google's guidelines on this matter…