EMO is a style of music.
this should help clarify...
"Emo" (or emocore) is short for "emotional hardcore." It is essentially a subgenre of hardcore punk music, tagged sometime in the mid-80's to describe a particular style of DIY bands wanting to escape the traditional standard of hardcore (Minor Threat era) and the violent scene at the time, giving way to a new wave of experimentation that would influence many bands to come. The term emo was derived from the fact that, on occasion, members of a band would become spontaneously and strongly emotional during performances. This term wasn't taken too lightly by some bands at the time, however.
Rites of Spring are thought to be the first band labelled with this term, although Hüsker Dü's 1984 album Zen Arcade is often cited as a major influence for the new sound. While many bands of this genre came to play seemingly different music altogether, their influences are always rooted to original emo - the Washington, DC style, and regional scenes that spawned from it. Though describing the sound can't replace listening to the bands yourself, notable characteristics of emo music today are: "loud-to-soft" (sometimes chaotic) dynamics, twinkly melodic breaks, frantic/abrasive shouting or screaming, angry/abstract/introspective lyrics, low-end production, and exceptionally energetic live shows.
The "DIY" punk ethic refers to the idea of 'doing it yourself.' Essentially, making and promoting music without major record label backing, and without any great level of "selling out". This means that you're not going to hear emo on the tv or radio, kids (ok, i found one exception). You'll find most of these releases on vinyl, and sold in independant record stories.
"Screamo" was a term coined later to describe the transistion of hardcore emo bands in the 1990's who started playing a louder, faster, more chaotic style, including constant screaming with fast, harmonized guitars. "Emo" and "screamo" are essentially meant to describe the same style of music. While screamo is more reserved for modern bands, emo is often used in reference to original emo, or the entire genre at a whole. "Emo Violence," sometimes misinterpreted as synonymous with "screamo," was a joke term created by In/Humanity in an attempt to describe themselves and a sound formed during the emo-screamo transition (a play on the words emo & powerviolence). While similar to both chaotic emo and grindcore, emo violence is known to sound lo-fi, with vocals pushed past the point of normal sound, with occasional spoken words or singing.
Most emo bands broke up by the 90's, examples are:
Rites of Spring
One Last Wish
Policy of Three
Portaits of Past
Modern screamo is constantly in revival, with examples including:
City of Caterpillar
Circle Takes The Square
Reversal of Man
The Spirit Of Versailles
You and I
Hassan I Sabbah
Welcome The Plague Year
Daniel Striped Tiger
Bravo Fucking Bravo
After the first wave of original emo bands had nearly dissapeared, emo's influence could be found amongst a slew of new indie bands (Sub Pop Records, for example), who were known to mix Fugazi (a highly influential band who'd come to feature Minor Threat/Embrace member Ian MacKaye and Rites of Spring member Guy Picciotto) along with post-punk elements. This is sometimes referred to as a "second wave" or "post-emo indie rock."
Examples of these post-hardcore/indie bands include:
Sunny Day Real Estate, Christie Front Drive, the Promise Ring, Mineral, Boys Life, Sideshow, the Get-Up Kids, Braid, Cap'n Jazz, Joan of Arc, Jets To Brazil, Texas Is the Reason, Death Cab For Cutie, Jimmy Eat World, At The Drive-In, Saves The Day, and Weezer.
Around and after 2000, a bastardisation* of emo terminology in turn created a fad in which had no relation with actual emo music. Showcased was a new scene of goth-like "emo fashion," stereotypes of crying and self-mutilation, and the words "emo" and "screamo" carelessly attibuted to pop-punk, post-hardcore, and dubbed-down metalcore bands alike. This is due pimarily to mainstream media, record companys, and the internet. Sometimes referred to as "popcore," "emobop," "mall emo," and "fake screamo" by those "in-the-know," none of these new bands beared resemblance to emo/screamo bands, and have hardly anything in common with them.
That being said, emo/screamo does not include the following bands:
AFI, Alexisonfire, A Static Lullaby, Alkaline Trio, All-American Rejects, The Ataris, Brand New, Bright Eyes, Coheed and Cambria, The Early November, Emery, Fall Out Boy, Finch, From Autumn to Ashes, From First to Last, Funeral for a Friend, Hawthorne Heights, Matchbook Romance, My Chemical Romance, Panic! At the Disco, Saosin, Senses Fail, Silverstein, Something Corporate, The Starting Line, Story of the Year, Taking Back Sunday, The Used, Thrice, Thursday, or Yellowcard.