What Is It?
The short and simple answer is that Emo is Hardcore Punk music with sensitive and emotional lyrics. The music is epitomized by post-Grunge, edgy rock with explosive energy mixed with sensitivity. Emotional lyrics about sadness, love and even anger are common lyrical themes. There is also a subculture of young people that are considered Emo. The casual Emo garb is faded blue jeans or slightly oversized work pants accompanied by a secondhand T-shirt with an out of place logo, bad Heavy Metal tees work well too. Good shoes would be Converse Chuck Taylor low-tops or old skool low-top Vans. The more extreme clothes of choice would include horn-rimmed glasses, cardigan sweaters, dorky polo shirts, pants that are a bit too short and hair that is spiky or messy in the back with straight cut bangs.
Where It Started?
Emo was born out of the ashes of the Hardcore punk band Minor Threat. They were a Straight Edge Punk band, meaning that they promoted clean living (no drugs, alcohol or sex combined with antiestablishment politics). When Minor Threat broke up in 1984, members formed other bands like Embrace, Rites Of Spring and later Fugazi, which were the first Emo bands. They still played hardcore punk music, but the lyrics were more expressive and emotional instead of being focused on politics and disenchanted youth (like most punk). Other early influences came from Husker Du and The Descendants and All.
How It Progressed?
As time went on, bands began toning down the Hardcore aspect of the music, making it much more appealing to the average listener. Christie Front Drive infused pop sensibilities like catchy melodies and hooks into the music and kept the Emo style lyrics. This formula would prove to be very successful and influential. After the hype of Grunge died down in the mid 90s, artists like Sunny Day Real Estate burst from obscurity into the spotlight and became a blueprint for the style. Weezer took it a step further and created a Pop-Punk / Emo hybrid.
Emo is one of the most widespread and hottest genres. Although it's moved far away from it's Hardcore beginnings, it still has it's independent spirit intact. That said, major record labels have taken notice and released albums by Jimmy Eat World, At The Drive-In and Weezer just to name a few, and many more are surely in the works. However, many newer bands are coming dangerously close to losing credibility by taking the music too far into the Pop/Punk realm of bands like Green Day or Blink-182.