The GMC Sierra 2500 and 3500 both share the "HD" tag, which stands for "Heavy Duty". These trucks are designed to be true beasts on the job site. That's not to say that the GMC Sierra 1500 is a slouch: it has great towing and payload capacities, after all. The Heavy Duty Sierra trucks are simply stronger and larger. They're designed with the job site in mind, while the Sierra 1500 is meant to be a refined mix of work potential and daily driver capability.