Jam bands are some of the most fun to attend in concert, and not just because of the audiences they attract. Regardless of what they record in the studio, a typical jam band concert song will involve about three or four minutes of the song with lyrics, another four or six minutes of extended quasi-improvisation, and a fade right into the next song.
The best jam bands (of whom the Grateful Dead are the first and still the best) are known and loved for their live recordings, not their studio ones.
Jam bands are not necessarily as improvisational as your typical jazz or blues artist, but that doesn't mean they're not good at it. | 
