"Emo" is not short for "Emotional." "Emo" does not mean Taking Back Sunday and Dashboard Confessional, despite what MTV has lead you to believe in the last few years. "Emo" is not sidebangs, tight pants, and male vocalists who sing like little girls about their failed relationships. "Emo" is not the use of diluted, meaningless metaphors and similes such as "My arms are like pinecones," and most definitely is not the rampant use of words such as "autumn," "heart," "knife," "bleeding," "leaves," and "razorblade."
I just thought I'd clear that up after all of these "definitions" in which I have encountered an unbelievable amount of people who try to pass off their blatantly false pretenses as fact, and are slowly infecting others with their high-horse, holier-than-thou bullshit. Because honestly, with your ridiculous definitions, Beethoven, George Gershwin, and Britney Spears are/was "emo bands."
Now, onto the real definition.
In the early 90s there was a movement in the hardcore genre that came to be known as "Emotive Hardcore," spearheaded by Rites Of Spring. Harder-core-than-thou kids, who swore by Dischord Records a la Minor Threat, actually coined the term "Emo" as something of a put-down for the kids who really liked Rites Of Spring, Indian Summer and this new wave of "Emotive" Hardcore bands. That's right, "Emo" was once not something kids called themselves. The field exploded outwards from there - Level-Plane Records has always been the most famous Emo label. Acts like Yaphet Kotto, I Hate Myself, Saetia, Hot Cross, A Day In Black And White, Funeral Diner, I Would Set Myself On Fire For You, You And I, and hosts of others came in the next decade. Most emo bands have since broken up, but there's still the occasional hold-out (again, the majority of Level-Plane Records' roster has been a procession of emo acts). Like most DIY hardcore/punk of the time, a majority found its way onto vinyl and not much else. Some people consider bands like Fugazi, and later Sunny Day Real Estate, a progression of emo, but personally, I don't quite follow that philosophy.
Often, more recently, this gets intertwined with post-hardcore, and understandably so - that's nothing to make an issue of, since well shit, at least it's close.
Since the late 90s, though, bands have been emerging in the vein of Taking Back Sunday, Dashboard Confessional, and the thousands of their clones. As far as I can tell, some lazy journalist somewhere, writing an article about them, decided "Well, f, no one knows what emo is anyways, so I'll call these bands "emo" - sounds more appealing than bubblegum pop rock..." and the spiral continued downwards into the current amalgomation of bands MTV has told everyone is "emo."
Somehow, people decided that "emo" meant "emotional," which is obviously bullshit, as 99% of bands make music to illicit emotion, which would make "emotional" a completely all-encompassing genre from classical to opera to pop to rap.
Hope that helps.
Taking Back Sunday, Senses Fail, and My Chemical Romance falls under the "horrible pop rock" genre, not the emo genre.
Rites of Spring is emo
emo bands...Rites of Spring,
Jets To Brazil
Life At These Speeds
Love Lost But Not Forgotten (also here)
Minus The Bear
The Mock Heroic