A hacker is someone who finds ways to get into computers and networks. This can be a person in a white hat (good) or a black hat (bad, although when people think hacker today, it is the bad guys they think of.
Hackers have been around since near the early days of computers. Until recently they were mostly good people.
The good guy hackers are people who are testing computers and networks to either find vulnerabilities or to test what all can be done with the computers and networks. These people will be going in to find where the openings are that people can get in so the software programmers can fix and close the holes and make the products more secure. They report what they find to make improvements. Another way to look at the good hackers is they are like a security company that tells you where the security weaknesses are in your house, like you have windows unlocked, or other places people can easily enter.
The bad guy hackers are the ones who make the news. They are trying to find ways to break in networks and computers to cause damage. When they find an opening to computer or network they need use that opening to go in and cause damage to the computer or network by messing up information or web pages or destroying web pages or information. These are the hackers that we do want around.
A hacker can be a good person or a bad person. When a IT person speaks of someone as a hacker find out if they mean good or bad, if a person on a street says hacker they probably mean bad. Being called a hacker in the past was a good thing, now usually it is bad.
Thanks Evelyn for the question.