Let me tell you that this is a common phrase nowadays between teenagers. The word "emo" is a short expression of emotional. Now, be carefull because for teenagers this is an expression that is related with the term "gay" or a person who is really strange. Belive me, I am a teenager and I have used that term!
I think the definition of emo varies from place to place. Where I live, an emo is an emotional person, who dresses in black, listens to 'emo music' (fall out boy, my chemical romance etc) and who is prone to self - harm.
Some friends of mine who live nearer Birmingham say that an emo is anyone who shows any of these characteristics.
Therefore, i think that different people see 'emo' as being different things. The usage of the stereotype can also be subjective to the characteristics of other people. If for instance, the majority of people in place X are chavs, then someone who is remotely emo will stick out more even if they display only one of the aforementioned characteristics.