These hackers are the bad guys that operate on the opposite side of the law to White-Hat Hackers. They may or may not have an agenda. In most cases, black-hat hacking and out-right criminal activity are not too far removed from each other. They do not have permission or authorization for their activities; typically their actions fall outside the law.
Black Hats are often individuals with extraordinary computing skills, resorting to malicious or destructive actions and are also referred to as crackers.