Dogs are much more susceptible to heatstroke than humans. One reason for this is, your dog wears his fur coat all year round. And while dogs do have sweat glands on their feet, they do not have them on the rest of their body. They rely on panting, a method of breathing out excess heat, to cool down their bodies. This method is not as affective as sweating.
Some breeds are much more susceptible to the heat then others.
Dogs with thick, double-coats have a harder time beating the heat. Also dogs with pushed back faces, such as Bulldogs, Pugs and Boxers, have smaller airways, and therefore have less of an ability to blow out hot air. The major cause of heatstroke in dogs is leaving a dog in a parked car. As you can imagine, with that fur coat on, your dog's body temperature rises very quickly. Exercising in hot weather is another common way heatstroke can occur.