Most Hindus believe in God but what this means varies in different traditions. The Sanskrit words Bhagavan and Ishvara mean 'Lord' or 'God' and indicate an absolute reality who creates, sustains and destroys the universe over and over again. It is too simplistic to define Hinduism as belief in many gods or 'polytheism'. Most Hindus believe in a Supreme God, whose qualities and forms are represented by the multitude of deities which emanate from him. God, being unlimited, can have unlimited forms and expressions.
God can be approached in a number of ways and a devoted person can relate to God as a majestic king, as a parent figure, as a friend, as a child, as a beautiful woman, or even as a ferocious Goddess. Each person can relate to God in a particular form, the ishta devata or desired form of God. Thus, one person might be drawn towards Shiva, another towards Krishna, and another towards Kali. Many Hindus believe that all the different deities are aspects of a single, transcendent power.
In the history of Hinduism, God is conceptualised in different ways, as an all knowing and all pervading spirit, as the creator and force within all beings, their 'inner controller' (antaryamin) and as wholly transcendent. There are two main ideas about Bhagavan or Ishvara:
Bhagavan is an impersonal energy. Ultimately God is beyond language and anything that can be said about God cannot capture the reality. Followers of the Advaita Vedanta tradition (based on the teachings of Adi Shankara) maintain that the soul and God are ultimately identical and liberation is achieved once this has been realised. This teaching is called non-dualism or advaita because it claims there is no distinction between the soul and the ultimate reality.
Bhagavan is a person. God can be understood as a supreme person with qualities of love and compassion towards creatures. On this theistic view the soul remains distinct from the Lord even in liberation. The supreme Lord expresses himself through the many gods and goddesses. The theologian Ramanuja (also in the wider Vedanta tradition as Shankara) makes a distinction between the essence of God and his energies. We can know the energies of God but not his essence. Devotion (bhakti) is the best way to understand God in this teaching.
For convenience Hindus are often classified into the three most popular Hindu denominations, called paramparas in Sanskrit. These paramparas are defined by their attraction to a particular form of God (called ishta or devata):
Vaishnavas focus on Vishnu and his incarnations (avatara, avatars). The Vaishanavas believe that God incarnates into the world in different forms such as Krishna and Rama in order to restore dharma. This is considered to be the most popular Hindu denomination.
Shaivas focus on Shiva, particularly in his form of the linga although other forms such as the dancing Shiva are also worshipped. The Shaiva Siddhanta tradition believes that Shiva performs five acts of creation, maintenance, destruction, concealing himself, revealing himself through grace.
Shaktas focus on the Goddess in her gentle forms such as Lakshmi, Parvati, and Sarasvati, or in her ferocious forms such as Durga and Kali.