So, what is "punk," anyway? Well, "punk" is used to describe "subsequent music scenes that share key characteristics with those first-generation 'punks,' and it is often applied loosely to mean any band with 'attitude' or 'youthful aggression.' The term is sometimes also applied to the fashions, ideology, subculture, or irreverent DIY" ("do it yourself") attitude associated with this musical movement."
As for some history... "Punk rock is an anti-establishment rock music movement which began around 1974-1975 (although transitional forms can be found several years earlier), exemplified by The Ramones, the Sex Pistols, The Damned, and The Clash."
And if you label someone a punk, you've got a lot of definitions to choose from according to TheFreeDictionary.com: 1) Slang a. A young person, especially a member of a rebellious counterculture group. b. An inexperienced young man. 2) Music a. Punk rock. b. A punk rocker. 3) a. Slang A young man who is the sexual partner of an older man. b. Archaic A prostitute.