The Devotionalistic Gods in Hinduism
While the old gods of the Vedas (Indra, Agni, Dyaus, Mitra, Varuna, etc. [note]) eventually were demoted by Hinduism to a position inferior to the Vedas themselves, in the Upanis.ads Brahman came to be conceived as the Supreme Being, or just Being — the One. According to the Dvaita Vedânta interpretation of the Upanis.ads, Brahman is a personal God, distinct from individual souls (atmans) and from matter.[1] Such a personal Brahman, whether formulated philosophically or not, comes to be identified in popular religion with either Vis.n.u or Shiva. Since different gods are thus proposed as the One God, Hinduism is an unusual kind of monotheism: it contains virtual sub-religions consisting of the devotees of Vis.n.u, the Vaishnavites, and the devotees of Shiva, the Shaivites. It is best to see this as an instance of the “multiplicity of explanations.”
On the other hand, the earlier (and perhaps more faithful) interpretation of the Upanis.ads is found in the Advaita Vedânta of Shankara [2], where Brahman is identical with the Âtman and is an impersonal Absolute beyond any devotionalistic Gods.[3] Since the personal Gods could all be seen on the same footing in relation to an impersonal Brahman, an attempt was made, we know not by whom, to tidy up things through the doctrine of the Trimûrti:
Brahmâ (the masculine form of Brahman), was a creator God in the Vedas (more or less identifiable with Prajapati), but he is actually not an important devotionalistic God. One story about Vis.n.u is that as he sleeps, dreaming the universe, a lotus grows from his navel and opens to reveal Brahmâ, who then creates worlds as he blinks his eyes. Brahmâ is awake for a kalpa, or a Day of Brahmâ, which is either 12 million years or 4 billion years. He then sleeps for another kalpa, a Night of Brahmâ, while all karma sleeps within him. After 36,000 Days and Nights, called the Life of Brahmâ (859 billion or 309 trillion years), Brahmâ dies, and all karma is annihilated. But then a new lotus grows from Vis.n.u’s navel, and another Brahmâ is born.[4]
The most important feature of the cult of Vis.n.u is the belief that he periodically causes himself to be born as a being in the world. He does this out of compassion, and this is probably responsible for his epithet as the “Preserver.” As Krishna says in the Bhagavad Gita (Chapter 4, Juan Mascaró translation, Penguin, 1962):
[7] Although I am unborn, everlasting, and I am the Lord of all, I come to my realm of nature and through my wondrous power I am born.
[8] When righteousness is weak and faints and unrighteousness exults in pride, then my Spirit arises on earth.
[9] For the salvation of those who are good, for the destruction of evil in men, for the fulfillment of the kingdom of righteousness, I come to this world in the ages that pass.
In the cycle of time within which we live, called a Mahâyuga (either 12 thousand or 4.3 million years), there are supposed to be ten Incarnations (or Avatars) of Vis.n.u. Nine have come already: 1) as the Fish, 2) the Tortoise, 3) the Boar, 4) the Man-Lion, 5) the Dwarf, 6) Parashurâma, 7) Râma (of the Râmâyâna), 
Kr.s.n.a (Krishna, of the Mahâbhârata), and 9) the Buddha. As the Buddha, however, Vis.n.u is supposed to have taught a deliberately false doctrine (which is how Hinduism always sees Buddhism), to destroy demons. The tenth Avatar, Kalkin, will usher in the end of the world (or the end of the Mahâyuga).
The most important feature of the cult of Shiva is perhaps his sexual complexity. This may come in answer to a difficulty that the chart of the Trimûrti may suggest. Each of the Gods is married, but the presence of a wife is a little awkward if the deity is to be considered the Supreme Being — there is no Mrs. God in the Old Testament. The solution is that the goddesses are really the female aspect of the God, not separate beings. Thus Shiva as a whole may be divided into male and female sides. This can be artistically represented either by showing Shiva as half male and half female, or by showing Shiva and Pârvatî locked together in intercourse. Much the same thing is shown through the union of the Shaivite fetish objects, the linga cone and the yoni ring or table.
This sets off a chain reaction of belief. The male side of the God is contrasted, as remote and detached, with the female side, which comes to be seen as the shakti, the active power and energy of the God. Then all goddesses are seen as active, powerful, and creative, and finally assimilated, more or less, into a sense of One Goddess, Shakti, who contains all power. This effectively eliminates the need for a creator Brahmâ, and gives rise to, virtually, a third sub-religion: Shâkta, or Tantrism (named after its texts, the Tantras). The Trimûrti thus might be refigured this way (though, evidently, it hasn’t been so far):
On the other hand, there are goddesses and there are goddesses. Some goddesses, like Pârvatî herself, are basically positive, protective, and reassuring; others and basically destructive, terrifying, and spooky. Kâlî is the most famous in that respect (thanks in part to the Hollywood exaggerations in Indiana Jones and the Temple of Doom). Shiva as a whole might be divided, not just into male and female sides, but with the female side divided into peaceful and terrible sides:
The most fearful manifestation of the cult of Kâlî was the murderous secret society of her followers, the Thugs (pronounce the “th” like a “t”; there is no “th” sound, as in “thin,” in Indian languages). Thugee was the practice of murder and robbery by the Thugs, who strangled travelers in their sleep. They saw these murders as offerings to Kâlî. The British decided this should not be tolerated, so they actually infiltrated the society and stamped it out.
A very nice statement of the complex of the devotionalistic Gods can be found by George Michell, in The Hindu Temple, An Introduction to its Meaning and Forms [University of Chicago Press, 1977, 1988]:
Innumerable gods and goddesses are found throughout the mythology and art of Hinduism, but the history of this religion at its highest devotional level is mostly bound up with the simultaneous development of two major cults — those of the male gods Shiva and Vishnu. A third cult is also of importance — that of the Mother or the Goddess — but is rarely seen in isolation, as the Goddess is essentially the consort of Shiva. These cults are synthetic in character owing to their evolution as amalgamations of many different minor deities. Shiva, Vishnu, and the Goddess are compound creations with a wide range of divine powers and richly paradoxical personalities. The majority of Hindus ally their beliefs with one or other of these cults, worshipping Shiva, Vishnu, or the Goddess as the highest principle. In devoting themselves to one of these three deities Hindus do not deny the existence of the other two, who are regarded as minor expressions of the divine power. Thus in the cult of Shiva, Vishnu is considered an unimportant aspect, whereas in the cult of Vishnu, Shiva is reduced to a secondary emanation. To the worshipper of the Goddess, the male gods are mostly passive and shadowy figures.
In general, the cults of Hinduism developed peacefully together and only rarely is there any evidence of friction or religious persecution. The reason for this mutual co-existence is to be found in the belief that the ultimate godhead lies beyond the divisions of cult and that the worship of Shiva, Vishnu or the Goddess leads inevitably to the same same goal. [p.23]
As I have noted, however, this latter is not quite right, only holding true for the School of “unqualified” Advaita Vedânta, not for “qualified” Advaita Vedânta or Dvaita Vedânta, where the “ultimate godhead,” Brahman, is a personal Deity identified with a devotionalistic God. Michell may have gotten the wrong impression because the “unqualified” doctrine is both more intriguing to Western investigators (already familiar with personal Monotheism) and more to the taste of most recent Indian philosophers. The real reason for the co-existence of the cults may just be that they identify each other’s Gods with aspects of their own. Or, it may simply be that Hindus are aware that there are many gods in their religion, and they do not regard the status of other devotionalistic Gods as different from the surviving gods of the Vedas.
Source