Emo, shortened for Emotional Hardcore, emerged in the 1980s as a form of Post-Hardcore in Washington, D.C. There are four "waves" of emo: first wave, second wave, third wave, and fourth wave emo.
The first wave of Emo was born right out of Washington D.C.'s hardcore-punk scene of the '80s with bands like Rites of Spring and Embrace being best known for kick-starting the genre.
The second wave of Emo, often called Midwest Emo for its geographic location, occurred in the '90s with bands like Sunny Day Real Estate, American Football, The Get Up Kids and Cap'n Jazz. Second-wave emo is considered to be a less raw version of what first-wave is.
The third wave of Emo started in the early 2000s where elements of Pop-punk became influential to the sound of Emo. Bands popular during this time were Hawthorne Heights, Taking Back Sunday and Fall Out Boy.
Fourth wave emo is referred to as the "emo revival," starting from the early 2010s. While a fine blend of second and third wave, the most recent wave has been heavily inspired by indie as well as American Football. Good examples of this genre would be Modern Baseball, Joyce Manor, Tiny Moving Parts, Mom Jeans, Jank, and Remo Drive.