different people have very different definitions.
what you are seeing and what people are bitching about is mtv's emo.
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It's short for emotional punk, just one of many sub genres/movements of punk
i.e. ska/pop/goth
Most people date its beginnings in early 90's d.c. punk scene, where bands would start to include more personal/emotional lyrics
It's really only today/mtv's version that you get these ultra whiney voice singers
but at the same time, they all get lumped together
people will say fugazi, at the drive in, american football, dashboard confessional are all emo.
It's come to the point where it encompasses so much that it has lost all relevant meaning and it's more of a *personal* definition.
You'll have your purists, elitists, ignorant and the uneducated. 
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Now with any music movement, it has its own sub-culture/standards consisiting of how to act, what to wear etc...
Again, everybody will have their own definition and once again what everybody see is the mainstream/mtv version...but that's what people are complaining about.