Most of the time, when someone says 'punk' they are talking about the music or someone that listens to that style of music. However, originally the term was a slur and meant one of the following (taken from Google's dictionary):
1. a worthless person (often used as a general term of abuse).
"you think any of these punks they got fighting today could stand up to Joe Louis?"
2. a criminal or thug.
"there's never been a better time to take our streets back from the punks"
US derogatory
3. (in prison slang) a passive male homosexual.
"he determined to be made into no one's punk"
4. an inexperienced young person.
"you were a nobody, a punk kid starting out"
Hope this helps!