There are 3 main kind of hackers. Black Hat Hackers are malicious hackers that intend to do damage by exploiting vulnerabilities that they discover. White Hat Hackers (ethical hackers) are normally employed to use their knowledge and skill for the good of discovering vulnerabilities and providing a solution for rectifying them. Grey Hat Hackers sit in the middle of Black Hat Hackers and White Hat Hackers, they will often discover vulnerabilities (normally without the permission of the data owner) and then will try and get rewarded from their discovery. Should they fail to get rewarded from their discovery they may then choose to reveal the vulnerability they have discovered to the wider population such as Black Hat Hackers.