I stole some pins from the noticeboard
And pressed them into my skin
And they spelled 'why?'
Why did they spell 'why'?
Because there weren't enough pins for 'oblivion'.
—Bill Bailey, "The Song Written From The Perspective Of A Young Man Who Works At Starbucks And Self-Harms"
Emo is short for Emotive Hardcore (a music genre), but has come to be used as a noun for a person who displays a fairly narrow range of emotion, all of it negative and most of it Wangsty. An emo's posts and commentary are all (mostly teen) angst and depression, consisting of litanies about things wrong with the world. In many depictions (and, let's face it, real life cases), the issues that the emo is 'suffering' through are really rather minor and / or insignificant in eyes of most people, their response to said issues is often seen as quite over-the-top and unnecessarily exaggerated, and they are often shown to be severely lacking in perspective.
As part of this lack of perspective, many depictions of Emos also have them fascinated by issues such as self-harming and suicide, not necessarily because they are suffering from any of the genuine problems or issues regarding depression or such that often drive people to such acts, but out of a misplaced and slightly worrying fascination with the 'glamour' supposedly inherent within these acts. As a result, the standard stereotype of an Emo is that of a self-pitying poseur, someone who adopts a superficial facade exploiting genuine problems and painful issues in order to appear cool or trendy and to belong to the sub-culture.
Needless to say, their "suffering" is more often than not found amusing by others, even other emos. Mocking emo posts is a common pastime on more cynical fora, and they are a favorite target of some trolls. One of the most popular Internet memes is an type of Image Macro showing sad or pitiable figure (e.g., a crying child, a mewling kitten, a 'sad-face' clown) with some variant on the caption, "Cheer up, emo kid!" In some cases, however, those who are genuinely suffering from psychological issues such as depression may be lumped in with those who are merely pretending to in order to appear part of the sub-culture.
Interestingly enough, in Elizabethan times, many young men posed as "melancholy" which translated to sad, troubled, fond of wearing black, and artistic, making this trope Older Than Steam.
There is also a music genre called Emo (short for "emotive hardcore"), the source of the term, due to the fact that it's perceived as the most popular genre of music among emo kids. Note also that the emo genre was originally a form of hardcore punk associated with displays of positive emotion, and only became what it is today when a popular band went from happy to depressed but still marketed themselves as emo, when they were more along the lines of straightforward pop punk. Within the music industry it's rapidly turning into a catch-all term for anything with male vocals that isn't either hip-hop, country, metal or plain ol' boy-band popcorn. Some Emo music really isn't that bad in the eyes of the mainstream, but the awful songs are more notorious and form a very rich well of Narm.
Note that thanks to A Darker Me, the "emoness" of an individual tends to be exaggerated online. Some folks have taken the mere possession of emotion that is not one of pure happiness to be "emo", greatly stretching the definition entirely. The key factor of emo is drawing attention to one's minor problems, not reacting to actual traumatic and seriously devastating events, dealing with negative emotions in a constructive manner, or keeping all negative emotions quiet and thus not actually drawing attention to themselves.
Not to be confused with comedian Emo Phillips, nor with Emo, County Laois. And especially not to be confused with Goths. See Emo Teen for the portrayal of the subculture in fiction, and Wangst for characters felt to push this to the extreme.
- think emo looks minus the emo attitude with a bit of pop-music and rave culture mixed in