There are some (alot) varsity club hippies that will ONLY listen to bands that have been labeled 'jam'. They refuse to try anything beyond Phish or other groups/artists that are similar. I have no problem with Phish, I dont really like them outside of a couple songs...but their fanbase (like DMB's, etc) is just sadly one dimensional.
My recently deceased band could even be lableled 'jam' for a song or two and a penchant for improvisation, freestyling, and varied setlists. But every song is diferent, just like with bands under the 'jam' umbrella. Every bands inspirations, goals, etc are different and when matched by talent/passion it makes something unique.
Terms like 'emo' or 'jam' attempt to lump too many bands into one category until they dont fit...Jeff Buckley sang with a high voice and was very EMOtional, but is he 'emo'? The Grateful Dead had origins in jug music, bluegrass, rock, etc and didnt record any songs that sounded like another in their catalogue...yet they noodle away and play for hippies, so theyre a 'jam band'. And, of course, Coheed&Cambria has a high voiced singer and songs that deal with EMOtional topics so they have to be 'emo'.