Hinduism as a religion is very clear that there is only one Supreme Being who is omnipotent, omnipresent and omniscient and is formless and without any characteristics. All things, living and non-living are in essence a part of this Supreme Being though they appear as people, trees, buildings and mountains because of an illusionary force called Maya.
In Hindu mythology the Supreme Being manifests itself as three different forms responsible for different functions. These forms have come to be known as Supreme Gods. Brahma is responsible for the Creation of the Universe. Vishnu is responsible for its Preservation and Shiva for the Destruction. After Destruction there is another Creation and the cycle continues. Each of these Supreme Gods has his consort. Brahma’s consort is Saraswati who is the Goddess of Learning. Vishnu has Lakshmi for his consort who is the Goddess of Prosperity and Shiva’s consort is Parvati, the giver of Shakti, which is best translated as life force. Vishnu has no children. Shiva and Parvati have two sons, Kartikeya and Ganesha. Though Kartikeya is elder, Ganesha, the God with the elephant head is more revered. In fact it has been ordained that no God can be worshipped unless offerings are first made to Ganesha. Also no work of any importance is undertaken until due respects are paid to Ganesha. Using his divine powers Brahma has created many celestial sages who play roles of varying importance in Hindu mythology. The most famous is Narada, who is sometimes the mischievous celestial messenger and at others the epitome of devotion. Another famous sage is Kashyap who is the father of the demi-Gods and Demons.
The demi-Gods are subordinate to the Supreme Gods, but both are often referred to as just Gods, so one must keep the distinction in mind. They represent the Pagan aspect of Hindu mythology, with demi-Gods representing the various natural forces. Indra is the God of Thunder, Varun the God of Sea, Yama the God of Death and so on. Indra is their king and rules over them with an iron fist. Their kingdom is Heaven. Subordinate to the demi-Gods are the deities of the planets, animals, rivers and so on.
The archenemies of the demi-Gods are the Demons. Both the demi-Gods and Demons had the same father, the sage Kashyap, but different mothers. Aditi, the mother of the demi-Gods was a paragon of virtue and hence the demi-Gods are supposed to be the embodiment of all that is good. However this is not always true. Diti, the mother of the Demons, was a shrewish woman; therefore the Demons are the evil ones. The demi-Gods and Demons are always at war, because the Demons covet the kingdom of Heaven. Whenever there is a new Demon king, he invades Heaven sure that victory will be his. But the Demons always lose in the end. The Supreme Gods, in particular Vishnu, directly or indirectly abet the demi-Gods because they do not want evil to triumph.
In addition to aiding the demi-Gods, Vishnu in his role of Preserver directly intervenes in subduing evil. He has appeared in various forms to conquer evil. These forms are known as Avatars or Incarnations. The most important of these are known as the "ten incarnations" which include those of Rama and Krishna. The tenth incarnation will appear just before the destruction of the world.
In the last section we discussed that Hindu mythology changed over time. This also applies to the structure of the Hindu pantheon. The structure described above is that which emerged after crystallization of the concepts. The various structures described in the earlier texts were quite different.
The dual layered hierarchy of Supreme Gods and demi-Gods is no coincidence. Ancient religions were necessarily pagan because divine explanations for the processes of nature were then required. Examples are the ancient religions of Greece, Rome and Egypt. Ancient Hindu religion was no exception. At one time the pagan demi-Gods occupied the highest position in the pantheon. In the case of the other religions, over a period of time the societies as they then existed underwent major transformations. The ancient religions became mere mythologies. New religions, such as Christianity and Islam, were adopted by the societies. At this time the processes of nature were clearly understood. There was no place for pagan gods, often quarrelling with each other. The realisation that God is a single benevolent entity who not only administers the universe but also guides the human race had set in. This happened in Hinduism as well but without the transformation in society. This led to the belief in a supreme being who manifests in three forms for three functions and who commands not only humans but the demi-Gods as well.