A style of band that may or may not understand the concept of a song but seems to care less. All songs are improvised nonsense. Imagine if you will, the theme song from the TV program Seinfeld being played for two hours in a continuous loop with drum and guitar solos as "accompaniment". As opposed to jazz fusion. Which is actually the same but with horns. The fans participate by playing hacky sack and smoking marijuana. This helps to distract their brains from realizing how terrible the music is. Sometimes there is singing but the bands have no real opinions on anything so it doesn't much matter.
Fans defend this with the oft uttered cliche: "They are all trained musicians and have degrees and stuff." The same defense taken to an extreme would be analogous to going to a dentist for a cleaning and coming out with dentures.
Possible reasons for the popularity of these bands would be that their fans do a lot of shitty drugs such as is the case in the "techno" scene.
I can't tell one of these jam bands apart from another. Are you sure this is a DIFFERENT jam band?
One is the Grateful Dead/Phish and the second is trying to sound exactly like the Grateful Dead/Phish.
"We need another Vietnam to thin out their ranks a little."