before this, my knowledge on hackers are those people who broke into a system, whether they to it for fun or for their own need.
which is the Black hat hackers, White hat hackers and also Grey hat hackers. What are they really?
The Back Hat Hackers =
Black Hat Hackers (also called "crackers"), are hackers who specialize in unauthorized penetration. They may use computers to attack systems for profit, for fun, or for political motivations or as a part of a social cause. Such penetration often involves modification and/or erasing of data, and is done without authorization and hence they should not be confused with “ethical hackers” (see white hat hacker).
They also may distribute computer viruses, Internet worms, and deliver spam through the use of botnets. The term may also refer to hackers who crack software to remove copy restriction methods: copy prevention, trial/demo version, serial number, hardware key, date checks, CD check (NO-CD) or software annoyances like nag screens and adware.
The Black Hat Conference is a major conference dedicated both to learning (and potentially furthering) Black Hat techniques and also to understanding and preventing them.
White hat hackers, also known as ethical hackers, or white knights, are computer security experts, who specialize in penetration testing, and other testing methodologies, to ensure that a company's information systems are secure. Such people are employed by companies where these professionals are sometimes called "sneakers."
Groups of these people are often called tiger teams or red teams. These security experts may utilize a variety of methods to carry out their tests, including social engineering tactics, use of hacking tools, and attempts to evade security to gain entry into secured areas.
The National Security Agency offers certifications such as the CNSS 4011. Such a certification covers orderly, ethical hacking techniques and team management. Aggressor teams are called "red" teams. Defender teams are called "blue" teams.
The Grey Hat Hackers =
A grey hat, in the hacking community, refers to a skilled hacker who sometimes acts illegally, sometimes in good will, and sometimes not.
They are a hybrid between white and black hat hackers. They usually do not hack for personal gain or have malicious intentions, but may or may not occasionally commit crimes during the course of their technological exploits.