I would describe them as a cross between punk and goth... If your going by looks. I mainly call people emo when their sad all the time, example: my sister is always sad for pretty muchy no reason, but she wears pink all the time. This other kid I know loves AFI, wears ripped up punky jeans, black shirt, black hair with bangs that cover up his right eye, but is suprisingly happy, so I usually don't reffer to him as emo.
Anywho, I thought the term emo originated from the interent. Sites like DeviantART and myspace seem to have a large population of them.