A stallion is a male horse useful for stud that usually has a thicker neck compared to mares and geldings, as well as a more muscular physique.
Although stallions are generally more fiery temperament than mares, this is not always the case. It depends very much on breeding, training, and each horse's own temperament.
Due to their nature, stallions should be handled and ridden by experienced horse-people, as a novice rider usually does not have the knowledge to control the horse during riding, and inexperienced handlers could be harmed or overpowered.
Information source: Wikipedia