Hindu Gods Chart. There are four main sects within Hinduism: 1. Shaivism (in which Shiva is worshipped as the main god); 2. Vaishnavism (in which Vishnu worshipped as the main god); 3. Shaktism (in which the female aspects of god are primarily worshipped); and 4. Smartism (in which six main gods are worshipped: Shiva, Vishnu, Shakti, Ganesh.
In a picture, personality traits / characters are depicted via a face. So a gentle character has a gentle face, an angry character has a red face, a frightening character is depicted as someone who has gorging eyes and probably a tongue coming out of mouth. When an artist wants to show the various traits/characters (read 'faces') of a Hindu god/goddess, there is a dilemma. A single face won't be able to cover all the traits. Hence, the artists chose to give the god/goddess the many heads - in fact many faces.