There is a community, a shared culture, of expert programmers and networking wizards that traces its history back through decades to the first time-sharing minicomputers and the earliest ARPAnet experiments. The members of this culture originated the term hacker.
There is another group of people who loudly call themselves hackers, but aren't. These are people who get a kick out of breaking into computers and phreaking the phone system. Real hackers call these people crackers and want nothing to do with them. Real hackers object that being able to break security doesn't make you a hacker any more than being able to hotwire cars makes you an automotive engineer.
There are people who apply the true hacker attitude to other things, like electronics or music but in the rest of this article we will focus the skills and attitudes of software hackers, and the traditions of the shared culture that originated the term hacker.