A music subculture that embraces a DIY ethic and few chords.
Punk music was originally founded as counter-culture to hippie idealism and the over-blown prog rock of the early 70's, being very nihilistic but also weirdly optimistic with it's anarchists utopias. After the 1990's, Punk has essentially died thanks to pop punk like Blink 182, becoming an inescapable commercialism music machine creating horrors like goth and emo. How ironic.
Since then, the few remaining Punks revel in how much they hate modern music on, well, any music forum on the internet. Though they keep away from the more pretentious forums like Pitchfork and hang out more on sites like AbsolutePunk. Many influential Punk fanzines such as Razorcake, Cometbus, Maximum Rock n' Roll, and Slug & Lettuce have online presence thanks to the inherently "anarchist" nature of the internet. It's also very likely that any anarchist that you meet on the internet is a "Punk", since that's SUPER PUNK YOU YUPPIE. Despite Punks looking like Punks, they are actually very excitable due to being pretty sensitive towards offensive jokes.
Thanks to 30+ years of history, there are also countless sub-genres of Punk with specific themes, such as Riot Grrrl focusing on hard-core feminism, and Crust Punk, which are essentially purist anarchists who are homeless forever so they become "crusty". Crust Punk is actually the most common form of Punk these days. There isn't much uniformity besides spiked color hair and trying to look tough. Predictably, there are numerous forums that go on how to be a true... whatever sub-genre and not to be a "poseur".
Punk grew really out of garage rock like The Kinks, mainly taking their three chord progression and raw, unprocessed sound. The word "punk" was first used to describe the 1960s garage band Question Mark & the Mysterians but it didn't really mean anything. Before 1977 there were many proto-punk/pseudo-garage bands like The Stooges, MC5, The New York Dolls, and The Modern Lovers. But it was The Ramones, Sex Pistols, and The Clash that really brought Punk to the forefront and made it an art in 1977, during London's social unrest of an economic downturn and Irish occupation of the 1970's.
The early 1980s founded hardcore punk with bands like Black Flag, Minor Threat, Bad Brains, and the Dead Kennedys. Goth grew out from here during the 80's. This made punk go more underground.
The 90s killed Punk with pop punk bands like Green Day, Sum 41, and Blink 182. Younger punks and the older punks saw them as sellouts for creating esentially every pop song from '99 on. It still lingers to this day with good reason. Emo grew out of pop punk to everyone's dismay.
There is much disagreement over whether punk is dead. There is also much disagreement over which bands are considered "Punk", since punk influences are heard in pretty much everything as every new musical subgenre is essentially rooted in punk.