A hacker is someone who finds and exploits security weaknesses in a computer system or network.

There are three types of hackers: white-, gray- and black-hat hackers.
White hats are the good guys, they use their skills to find and fix security issues in systems with the permission of others to make everything more secure.
Gray hats are in the middle, they try to find weaknesses in the system without permission, but they don’t necessarily have bad intentions, although they may ask for something in return if they find something.
Black hats are the bad guys. They hack only for their own gain, to blackmail or harm others and commit various forms of cybercrime.