Sweet god...the amount of definitions that should be saved for the avril lavigne fans...
People use the term emo music as a narrow minded attempt at attempting to categorize emotional music as 'everything that sounds like fall out boy'. It ain't. Here are the genres/musicians that could be referred to as 'emo', 'emo orientated' or perhaps 'crap' if you don't like it.
1980s hardcore/soulful punk: People argue that this laid the base for most 'emo' and it does share very similar qualities to more commercial forms we have today. Catchy guitar lyrics, rough lyrics dealing with love, unhapiness, anandonment, hatred, disullision, longer running times ( unlike all other forms of hardcore punk at the time ) etc. I would cite Husker Du's Zen Arcade as the best example of this era, but Rites of spring are another brilliant band.
Early 90s/90s screamo: As an off-shoot of emo this is included. Bands in the early 90s took emo on to more fast, distorted, screamed and abstract horizons. You can easily recognise alot of this music by these qualities but running times varied alot ( indian summer's track 12 ran for 17 minutes, where as jerome's dream songs ran for much shorter ), lyrics may be intelligible at parts or not atall, there may be huge intros or guitar riffs lasting the majority of the songs thrashed and or melodic. Indian summer, jerome's dream, joshua fit for battle, swing kids are examples. Modern pop punk should not be confused with this genre under pain of death.
mid 90s/90s emo: More melodic and slower than previous forms, 1990s saw the rise of 'indie-styled' emo outfits. Best recognisable by whispered lyrics, drowning guitar riffs, complicated and emotional song meanings. Exceptions to this such as Jimmy Eat World and get up kids resemble the modern pop punk movement far more with more pop orientated fast guitar lyrics, but they are far slower than the previous forms and overall emo followed an indie style. Texas is the reason, Christie front drive, Sunny Day real estate are examples, with jimmy eat world, get up kids, lifetime following faster formats and lyrical styles.
2000s: Pop punk sets the precident in this era with catchy guitar lyrics, simple chorus structure, fast pace, easy to understand songs etc. There does seem to be a modern divide between bands choosing harder guitar riffs and even screamed lyrics ( silverstein, hawthorne heights, mcr, fall out boy ) and others following more traditional pop formats of gentle guitar riffs and softer singing( simple plan, Taking back sunday ). Mcr, fall out boy, silverstein, hawthorne heights, a thorn for every heart, the used, funeral for a friend are examples.
Late 90s/2000s Metalcore: Softer metal dealing with similar topics to emo - love, death, hatred, unhapiness etc. Often screamed/loud lyrics, hard/pronounced guitar riffs, hard drumming. Bullet for my valentine, bleed the dream, it dies today are examples.
2000s screamo: Perhaps as a front against the pop punk movement, bands tend mirror earlier, more hardcore forms of 'emo' with screamo being the most popular. Many little known, quite popular or even established screamo bands exist today following the general format of screamo in the early 90s. Funeral diner, circle takes the square, iwrotehaikusabout cannabilisminyouryearbook ( I had to space that ), emo summer ( who classify themselves as extremo...) are the best examples.
Confused yet?? You should be if you like this stuff, the world just doesn't get people like me and you and nothing should make sense.
Scene Pete: Woooooo I'm listening to emo music, mcr totally kick ass. Black parade pwnzorz.
Emo Dan: Leave me alone I'm listening to Jerome's dream, I want my ears to bleed so I feel pain.
Scene Pete: Zommmzggzgzzbbq!!1111 Screamo??! That is so cool, I want to listen to it.
Emo Dan: Scene cunts.