emo - short for "emotional." Emo is a broad title that covers a lot of different styles of emotionally-charged punk rock.
The summer of 1985 become known as "Revolution Summer" when a new wave of rock-tempo, melody based, sung-vocal bands forms out of the DC punk musician pool with diverse rock sounds - Three, Gray Matter, Soulside, Ignition, Marginal Man, Fire Party, Rain, Shudder to Think, etc. Few bands retain the fast hardcore punk-based sound with the new vocal approach, Dag Nasty being the notable exception.
Minor Threat's singer, Ian MacKaye's, sings for a band called Embrace, whose lyrics are emotional and deeply self-questioning, but still clear and unambiguous. Musically, the group writes midtempo, somewhat jangly music with a lot of pop guitar hooks. MacKaye's vocals retain his trademark bold enunciation, with only occasional sparks of emotive delivery.
1986, some bands begin to focus on the "emo" element itself. The Hated in Annapolis seem to be the first post-Rites of Spring to do this. Shortly thereafter, Moss Icon appears in in the same town. Moss Icon strips the "emo" element down to the core, and adds a great deal of intricate, arpeggiated guitar melody with a strong focus on loud/soft dynamics. The vocals, too, break new ground by building up to actual top-of-the-lungs screaming at songs' climaxes.
Moss Icon, as a relatively well-known band that toured some, introduces the punk scene to music that has core emphasis on emotion instead of punk energy. As such, I consider them the starting point for the emo movement, not Rites of Spring as is more commonly asserted. Later emo bands draw heavily from the Moss Icon dynamics, guitar style, and vocal delivery.