A star is a white spot on the horse's forehead. All horses have a whorl of hair in the center of their forehead, and sometimes a white marking forms around it. Stars are most often circular or close to circular, but can occasionally be closer to a diamond shape.
A rather "smudged" star on a black horse.
This bay also has a star. The white hairs around his muzzle are, like greying in humans, a sign of age.
A large, diamond shaped star on a Quarter Horse mare. (Often considered a very desirable marking.