Black hat hacking is the act of using computer security hacking for illegal means, usually to compromise high value computer systems. It is the opposite of white hat hacking.
Black hat is what popular culture associates with the word "hacker"; someone using computer skills to break through the security controls on systems they do not own usually in order to steal, modify, or delete data, spread malware, or commit other acts of cyber crime.
In contrast to white hats, black hat hackers are typically motivated by either personal financial gain or for political motives and often operate in secrecy behind anonymous pseudonyms.