Though both breeds look similar there are defining differences. The basic ones are these.

Shires originated in England. Clydesdale's in Scotland.

Shires tend to be heavier -bodied and were taller. Clydesdale's have more white markings, more colours and more feathering(long hair) at the bottom of their legs.

Shires have longer, leaner heads and big ears. Clydesdale's have a broader face and forehead and muzzle.