A black hat is the villain or bad guy, especially in a western movie in which such a character would wear a black hat in contrast to the hero's white hat. The phrase is often used figuratively, especially in computing slang, where it refers to a hacker that breaks into networks or computers, or creates computer viruses.
Black Hat Hackers, who may also be known as Crackers, are Hackers, who specialise in unathorized penetration of information systems. They may use computers to attack systems for profit, for fun, or for political motivations, as part of a social cause. Such penetration often involves modification and/or destruction of data, and is done without authorization. They also may distribute computer viruses, Internet Worms, and deliver spam through the use of botnets.