Emo is basically a style of music that can be considered as part of the rock music. This kind of music is very expressive in nature and has confessional lyrics. The origin of this kind of music dates back to the 1980's.
It is known as emo because it stands for Emotional Hardcore or emcore and was an outcome of the punk movement that took place in Washington. It was a reaction to the excessive violence in the society. Bands such as rites of Spring and echo can be credited for its invention.
This term became public during 1985's. In the beginning, this style of music was looked down upon by everyone by it gradually became very popular in the youth around the world. However, it was pretty short spanned. As it lasted for only a few years. Most of the bands following this kind of music broke up the next year.