A genre of music beginning in the mid 70s, with roots in NYC (the Ramones) and England (the Clash, Sex Pistols). The music is generally fast with an emphasis on the bass, and the lyrics are generally leftist.
It went through a revival in the mid 80s, with a new emphasis on melodies, rather than rhythms.
It underwent a second revival in the early 90's, helped along by early Green Day (American idiot is not Punk. dookie is.) and the Grunge scene, and the like. Some of the best bassists and lyricists ever have been associated with the punk rock scene. The major label for punk rock today is Epitaph, which has signed bands like NOFX, Rancid, Operation Ivy, Black Flag, and the distillers. All the above keep the origional punk spirit. Bands that do not but are classified as such anyway by the dipshits at major record labels include: Good Charlotte, Blink 182, and Sum 41.
The generally accepted culture of "Punk Rockers" Is as follows.
clothes: typically music or anti-establishment oriented, ripped, faded, etc. Pants are loose (but don't have 50'leg openings) to tight (but not emo tight), and are similarly faded jeans or twill pants. Plaid pants are generally considered punk as well. Boots: Chuck Taylors, tough shoes (doc martens, combat boots)
Attitude: Fuck the cops, Fuck the government, etc.
See SLC Punk.
That bugger that just kicked the bollocks out of that poser is a real Punk.