Here is a bit from the bio from The Rock and Roll Hall of Fame, which inducted the band in 2003:
Quite simply, the Clash were among the most explosive and exciting bands in rock and roll history. They played a major role in creating and defining the punk movement. If the short-lived Sex Pistols were glorious nihilists, then the Clash expressed punk’s impassioned political conscience. Their explosive, uptempo punk-rock manifestos were unleashed with pure adrenaline and total conviction.