A jam band is a musical group whose concerts and live albums substantially feature improvisational "jamming." Typically, jam bands will play variations of pre-existing songs, extending them to improvise over chord patterns or rhythmic grooves. Jam bands are known for having a very fluid structure, playing long sets of music which often cross genre boundaries, varying their nightly setlists, and segueing from one song into another without a break.
Blues Traveler performing in 2008
Phish is an example of a jam band.
The Derek Trucks Band
Grateful Dead's Jerry Garcia and Mickey Hart performing on 11 August 1987 at the Red Rocks Amphitheatre near Morrison, Colorado
Grateful Dead was an American rock band formed in 1965 in Palo Alto, California. The band is known for its eclectic style, which fused elements of rock, blues, jazz, folk, country, bluegrass, rock and roll, gospel, reggae, and world music with psychedelia, the improvisation of their live performances, and its devoted fan base, known as "Deadheads". According to the musician and writer Lenny Kaye, the band's music "touches on ground that most other groups don't even know exists." For the range of their influences and the structure of their live performances, Grateful Dead are considered "the pioneering godfathers of the jam band world".
Grateful Dead in 1980. Left to right: Jerry Garcia, Bill Kreutzmann, Bob Weir, Mickey Hart, Phil Lesh. Not pictured: Brent Mydland.
The Mantra-Rock Dance promotional poster featuring Grateful Dead
Grateful Dead performing at Red Rocks Amphitheatre in 1987
Bob Weir playing his Modulus G3FH guitar in 2007