There is no such thing as Emo.
In the early 80's the Slash Magazine writer known as "Kick Boy Face" proclaimed that there was no such thing as "New Wave." His argument was that "New Wave" was just a more commercially viable term than "Punk." He was in fact correct, and onto a future of abuse of terminology in the music industry. Punk was so reviled in the time of it's origins that no one outside of the scene wanted to be associated with it. So instead the mainstream created the term "New Wave."
Today, in a post "Never Mind" world, we have the opposite. Now the underground is trendy, and the mainstream, instead of distancing themselves from it, want to somehow find ways to include themselves, and their unproven bands, with it. Apart from appropriating the culture of the underground by copying it's fashion, one such way is to steal it's terminology. When The Mars Volta came out Spin Magazine proclaimed them "pioneers of screamo." The only problem with this being that they didn't play "Screamo," that the term had been used to label bands as long before as the early 90's such as "Nation Of Ulysses," and that it never existed in the first place. Screamo was an accompanying term to Emo, which never existed either.
WHERE DID EMO COME FROM?
In the early years of Punk the style was transitioning rapidly. It went from the almost bubble gum sound of The Ramones, to the snarl of The Sex Pistols, to the rock/reggae/ska like sounds of The Clash. In America we saw another transition with bands like Black Flag, and The Teen Idols playing what became known as "Hard Core." This has all been documented in documentaries like "The Decline of Western Civilization," and more recently in the DC punk scene documentary "Salad Days," about the DC punk scene from 1980-1990.
As a dynamic, and ever changing form of art punk continued to evolve. Bands like Siouxsie and the Banshees, Bauhaus, and others were playing what became known as "Post Punk," and in DC bands like Marginal Man, and Rites of Spring began playing more emotional, or emotion based Hard Core Punk. There reaction to this was swift, and unfriendly among some in the scene, and the term "Emo Core," or "Emo," was invented as a slur against the transition by hardcore kids who didn't approve of emotional messages in Hard Core music. Emo was a slur, and nothing more. The music that followed had all the transitional changes from Hard Core that Post Punk had made from Punk so everyone in the underground scene has come to a general consensus that "POST HARDCORE," is the actual sub genre classification for this kind of music.
WHY IS IT IMPORTANT?
Distinguishing the sub genre of Post Hardcore, from it's origin sub genre of Hard Core is important for the same reason distinguishing Hard Core from Punk, and Punk from Rock is. The changes, and time periods in which they happened are important to the history of rock music, and they also give credit, and justified respect to the musicians who pioneered such movements. When we allow this historical record to be trampled on by the mainstream giants, by allowing sub genres to be mislabeled we allow the history of musical, and social movements to be erased, and forgotton. Also, not for nothing, but by using the term "Emo" you are basically just advertising that you know nothing about music, and shouldn't be taken seriously at all.
SO WHAT IS BEING PLAYED, AND WHAT IS THE FASHION OF THESE GENRES?
Thanks to misuse of the term, and the complete lack of musical knowledge of the people using it, this is a bit more complex. It seems that every band that sings about anything emotional these days is labeled as "Emo." The first bands to really have this label thrown on them were the previously mentioned Rites of Spring, and Marginal man. These bands were in fact the first wave of what is commonly known as Post Hardcore. Later came bands like Fugazi, Cap'n Jazz, and Native Nod, among others. This sound can still be categorized as Post Hardcore.
Bands Like Joan of Arc, American Football, are more appropriately labeled under "Indie," a term that is in itself controversial, but for the most part accepted. Other acceptable terms are "Math Rock," or "Art Rock," depending on the band being discussed. MUCH later the mainstream started using the term for bands like My Chemical Romance, and such. These were just pop rock bands in costume so to speak. That is they played whatever the mislabeled pop rock known as "alternative" had become, of which most bands labeled as such didn't play. Nirvana was a grunge band, as opposed to Pearl Jam which was "alternative," which was actually just pop rock with a different mentality than Pop Rock had been used to having.
For some reason whenever people talk about "Emo," they tend to relate it to Goth fashion. I think a lot of this is due to the mislabeling of Pop Rock bands like My Chemical Romance, and the fashion they embraced to cover up the lack of depth they possessed. So all the face painting, dark clothes wearing poser kids using the term "Emo," are actully just Goth kids who don't listen to Goth music. Actual "Emo," or rather, Post Hard Core fashion is what became pimped out by the mainstream as "Hipster" fashion. "Hipster" being an older term, and fashion adopted by hardcore kids in opposition to the Punk Rock uniform, and the term adopted by the mainstream to, in my opinion, do nothing more than belittle the social, and political message of the scene. That is to say once someone got political you could attempt to silence them by just saying "what are you a hipster?"
So there you have it kids. Proof that there is no such thing as "Emo." Now, if we could just stop using the term, and properly labeling bands maybe, just maybe the kids today would have a better understanding of ACTUAL music history, and culture. Music history being something that the mainstream seems to have a real problem understanding past 1977. The reason for which being that once rebellion became marketable they bastardized it as much as they could, and ignored the underground until 1992 when Nirvana's Never Mind came out, and they couldn't ignore it again until after the Poppy Punk explosion of the mid 90's. So, now that you have been told, don't use the term "Emo" again unless you are using it in it's proper function as an insult. Doing otherwise will result in eye rolling, dismissal of any opinions to come after, and perhaps even laughter at your expense, or worse...