Hinduism is well known for its many gods and goddesses, images of which populate temples and home altars. This would make it easy to simply label Hinduism as polytheistic (believing in many gods). However, most Hindus would say they actually believe in only one God (monotheism) and many Hindus would further say that God pervades all things and is embodied in the universe (monism).
The Many Gods of Hinduism
The oldest and most sacred texts of Hinduism, the Vedas, are chiefly concerned with mythologies and rituals related to a number of deities, most of which are identified with aspects of the natural world. The most prominent gods in this early period, known as the Vedic gods were Indra (sky god), Agni (god of fire), and Soma (god of a ritual drink of the same name).
Later, the Trimurti (often translated "the Hindu Trinity") became more central, consisting of Brahma (the creator), Vishnu (the sustainer), and Shiva (the destroyer).
Today, Brahma is rarely worshipped but Vishnu and Shiva remain of primary importance, along with the Goddess (Devi/Shakti), Surya (sun god), and Ganesha, the elephant-headed son of Shiva. The gods may have many manifestations and avatars (incarnations), the most popular of which are Krishna and Rama (avatars of Vishnu). There are also a myriad of local community gods worshipped across India.
Devotion to these various deities is based primarily on one's region and needs, and even when devotion is given to only one, the existence of others is acknowledged. Hindu worship virtually always involves sculptures and images, to which offerings are made and rituals are performed.
The One God of Hinduism
Despite these polytheistic elements, however, the vast majority of Hindus believe that the gods are various forms of a single Supreme Being or Ultimate Reality known as Brahman. This belief is rooted in in philosophical Hindu texts such as the Upanishads. Some schools of Hindu philosophy focus more directly on this Supreme Being than individual gods.
"For the individual there is one supreme God, however conceived or named.... One author has rightly pointed out that one could spend a lifetime in India and never find a 'polytheist' in western terms, because even an unlettered peasant who has just made offerings at several shrines will affirm that 'Bhagvan ek hai', God is one." (Penguin Handbook of the World's Living Religions, p. 285)
"The divine - which Hindus may perceive as brahman, or Devi, or Krishna, or another - manifests in many names and forms, and, in this sense, the many are an expression of the One. Yet both remain important. Hinduism is both polytheistic and monotheistic. Both the many and the One have a place within it, and within the everyday experience of most Hindus." (Hinduism: A Very Short Introduction, p. 55)
"Hinduism worships multiple forms of the one God." (OM, an American Hindu organization)
"According to the tenets of Hinduism, God is one as well as many." (HinduWebsite.com)
"Hindus believe in monotheistic polytheism, rather than polytheism." (The Hindu Universe)
"Even though Hinduism is mistakenly regarded by many as a religion having many gods namely, polytheism, yet truly speaking Hinduism is a monotheistic religion." (Sri Swami Chidanda)
Some Hindus use the word Ishvara (Sanskrit, "Lord") or Bhagavan to refer to a personal God who can be known (unlike the abstract concept of Brahman) and more specifically to the believer's favorite or chosen god (ishtadeva). Views differ as to the relationship between the two: whether the personal or abstract is the superior representation of the Divine or if they are one and the same. ("Ishvara", Encyclopaedia Britannica)
The Divine in All Things
Some gurus and Hindu schools of philosophy have taught that the one Ultimate Reality or God pervades all beings and all things: God is embodied in the universe and/or is identical with the soul or self (atman) of all sentient beings. "Hence, in serving others we are - quite literally - serving God." (Introduction to World Religions, p. 193)
"All beings, from the smallest organism to man, are considered manifestations of the Divine (existence, pure being, light of consciousness) or reflections of the Divine's qualities, depending upon the school of thought." (Hindu American Foundation)