The original meaning of Punk was to be the submissive participant in the prison shower block. Aka to get fucked in the ass.
Then it was the name of an american music fanzine giving coverage to the very earliest punk and proto-punk bands.
This then became the name of that music scene and the name stuck.
Punks will typically be thinking in a different way to the rest of society for one reason or another and the scene ranges from extreme right-wing bands with neo-nazi politics such as Skrewdriver to the left-wing anarchic punk of Subhumans (bloody good live) or Crass. Most punk tends towards a more left-wing agenda, whilst not quite begging for a social revolution, rather, evolution such as The Clash.
Whilst the original scene was open and supportive (it would have died without reggae) it has now become elitist, petulant and to an extent soulless. Benji Madden said that Punk gave him a place to go where he felt like he belonged...which really isn't what it's about. You don't really 'become' punk it's just something that is there with you, I suppose.
Sub-genres of punk include: Street, Oi!, Hardcore, Pop-Punk and Ska-Punk.
Unfortunately, in the 80s the US scene spawned a couple of extreme bullshit punk offshoots i.e. Emo and Straightedge. Neither of which deserve your time.
Here are some punk bands: The Clash, Subhumans, Dead Kennedys, Rancid, Guttermouth, The Unseen, NOFX, The Damned, Sex Pistols, Sham 69, Operation Ivy.
Punk won't die, but you will.