The Mass media now uses the term hacker very synonymously with criminals who uses programming and mastery over aspects of the computer system and usually has illegal means and ends to do the task.

To differentiate the mass media used term and the original term a new plethora of definitions have emerged. These are white hat hacker, grey hat hacker, blue hat hacker, black hat hacker, cracker, script kiddie, Google hacker and so on.

White hat hacker is a person who hacks for altruistic reasons, Black hat hacker does it for dubious reasons and the Grey hat hacker is in between with an ambiguous set of ethics.

Blue Hat is hackers that are referred when a testing is done on particular software before it is launched. These are security experts or consulting firms looking for exploits in the software so that it can be closed before the launch.

More Glossary Terms Explained here