My understanding is that modeling is done to advertise product or service. There can be male or female model but in this article I will refer female models as they are the most sought after (even shaving cream ads feature female models). Wikipedia defines a model as:
A model (from Middle French modÃƒÂ¨le), sometimes called a mannequin, is a person who is employed for the purpose of displaying and promoting fashion clothing or other products and for advertising or promotional purposes or who poses for works of art.
Again from Wikipedia
The British Association of Model Agents (AMA) says that female models should be around 34-24-34 in (86-61-86 cm) and at least 5 ft 8 in (1.73 m) tall.