Punk is an aesthetic centered around the punk rock movement of the 1970s that followed in the footsteps of the Hippie movement. While the Hippies were more about peace, love, and harmony; the Punks were loud, abrasive, acerbic, and went out of their way to do whatever they could to offend members of the status quo. Many preached for anarchism, far-left politics (Punk in the UK was a direct reaction to the Margaret Thatcher-era of politics at the time, which was hardline right-wing, after all), and spitting in the face of the Establishment by rejecting the prevailing capitalist philosophy of the time.
Punk has evolved over time to be a symbol of rebellion, whether small-scale (teenager vs parents) or large-scale. Not all variants of punk are visually similar to the original 1970s American Punk (such as Steampunk) but contain similar messages about the potential for society’s decline. Aesthetics with the ‘-punk’ suffix generally share an attitude of defiance and/or rejection of mainstream society and, but there are some which are completely unrelated.
On the other hand, some people enjoy the visual aspect of 1970s original punk, and reject the philosophy, but most actual punks would refer to these people as posers. A popular example of this is the 1998 film SLC Punk!, which specifically made fun of people who wore the fashion but didn't adhere to the philosophy, with main character Stevo (played by Matthew Lillard) famously saying towards the beginning of the film:
Posers were people who looked like punks but they did it for fashion. And they were fools, they'd say "anarchy in the UK." What the fuck's that? Anarchy in the UK. What good is that to those of us in Utah, America? It was a Sex Pistols thing. They were British, they were allowed to go on about Anarchy in the UK. You don't live your life by lyrics.
Punk rock (or "punk") is a rock music genre that emerged in the mid-1970s in the United States, United Kingdom, and Australia. Rooted in 1960s garage rock and other forms of what is now known as "proto-punk" music, punk rock bands rejected perceived excesses of mainstream 1970s rock. They typically produced short, fast-paced songs with hard-edged melodies and singing styles, stripped-down instrumentation, and often political, anti-establishment lyrics. Punk embraces a DIY ethic; many bands self-produce recordings and distribute them through independent record labels.
The term "punk rock" was first used by American rock critics in the early 1970s to describe 1960s garage bands and subsequent acts understood to be their stylistic inheritors. When the movement now bearing the name developed between 1974-1976, acts such as Television, Patti Smith, and the Ramones in New York City, the Sex Pistols, the Clash, and the Damnedin London, and the Saints in Brisbane formed its vanguard. As 1977 approached, punk became a major cultural phenomenon in the UK. It spawned a punk subculture expressing youthful rebellion through distinctive styles of clothing and adornment (such as deliberately offensive T-shirts, leather jackets, studded or spiked bands and jewelry, safety pins, and bondage and S&M clothes) and a variety of anti-authoritarian ideologies.
While a lot of punk fashion featured tartan plaid pants, band shirts, leather jackets, mohawks, and multi-colored hair, but the key element of Punk was the DIY philosophy (something that the Grunge aesthetic inherited from them in the 90s) of wearing clothes either bought at thrift stores or handed down from older family members and modifying it to make it look more anti-establishment.