The most obvious physical differences between a hen and a rooster are the larger, more pronounced comb and wattles on the rooster's head as well as the rooster's thicker legs and well-developed spurs. Hens have all these features, but they're much smaller and less developed than on a rooster.
The fleshy protrusion on top a chicken's head is called a comb. Chickens also have bare skin hanging from the sides of their chin called wattles. The comb and wattles on most chickens are red, though they vary in shade between individual animals and are usually a much deeper red in roosters. The comb and wattles on roosters are also much larger, with the comb standing taller on the head and the wattles hanging lower from the chin.
While roosters and hens may both have spurs, the spurs on hens are generally tiny or barely noticeable. Roosters have long, curved spurs on the insides of their thick legs. The spurs get larger as they age. Roosters use these spurs to defend their flock against predators and other roosters.
The feathers between the back and neck on roosters end in a point, whereas those feathers on a hen are rounded. Roosters also tend to be more brightly colored, with darker blue, green and black iridescent feathers.