An eye stye is simply an infection or inflammation of one of the oil or sebaceous glands that lies along the edge of the eyelid. Eye sties appear as painful red lumps and then develop heads of whitish pus. They are not contagious and cannot be passed from person to person.
Children tend to develop sties more often than adults, probably because children's glandular secretions are more erratic, especially during puberty, and children tend to be less careful about keeping their hands clean and away from their eyes.