The Hindus believe in many gods and goddesses. At the same time they also believe in the existence on one Supreme God, whom they call variously as Paramatma (Supreme Self), Parameshwar (Supreme Lord), Parampita (Supreme Father). Iswara, Maheswara, Bhagawan, Purusha, Purushottama, Hiranyagarbha and so on.
God is one, but also many. He manifests Himself in innumerable forms and shapes. As Purusha (Universal Male), He enters Prakriti (Nature, Matter or Divine Energy) and brings forth the numerous worlds and beings into existence. He upholds His entire creation with His unlimited powers.
He is both the Known and the Unknown, the Being as well as the Non-Being, Reality as well as Unreality. As the Unknown, He is rarely known and worshipped for difficult and painful is the path for those who choose to worship Him as the Unmanifest (The Bhagavad-Gita XII.6).
He exists in all and all beings exist in him. There is nothing other than Him, and there is nothing that is outside of Him. He is Imperishable, unknowable, immortal, infinite, without a beginning and without an end. All the same when worshipped with intense devotion and unshakeable faith, He responds to the calls of His devotees and comes to their aid and rescue.
All the gods and goddess are His manifestations only. In His female aspect He is Shakti, who as the Divine Universal Mother assists the whole creation to proceed through the process of evolution in Her own mysterious ways.
The relationship between man and God is purely personal and each can approach Him in his own way. There are no fixed rules and no central controlling authority on the subject of do's and don'ts. There are of course scriptures and Smritis but whether to follow them or not is purely an individual choice.
The concept of monotheism is not new to Hinduism. It is as old as the Vedas themselves. References to One indivisible and mysterious God are found in the Rigveda itself. The concept is the central theme of all the Upanishads in which He is variously referred as Brahman, Iswara, Hiranyagarbha, Asat etc.
While the students of Upanishads tried to understand Him through the path of knowledge and there by made it the exclusive domain of a few enlightened persons, the bhakti marg or the path of devotion brought Him closer to the masses. The One Imperishable and Ancient Being was no more a God of remote heights, but down to the earth, ready to help His needy devotees and willing to perform miracles if necessary.
The rise of tantric cults added a new dimension to our understanding of Him. To the tantric worshippers the Supreme Self is the Universal Mother. Purusha is subordinate to Her and willing to play a secondary role in Her creation. By Himself He cannot initiate creation unless He joins with His Shakti.
On the abstract level He is satchitananda. Truth, Consciousness and Bliss. He is the inhabitant of the whole world. There is nothing that is outside of Him or without Him. He exists in the individual being as Atman, the Enjoyer who delights in Himself, without undergoing any change, but willing to participate in the cycle of births and deaths and bear witness to all the illusions of life.
He can be realized in many ways, which broadly fall into three main categories: the path of knowledge, the path of devotion and the path of renunciation. Of this the middle one is the best, the first one is very difficult and the third one requires immense sacrifice and inner purification. In the Bhagavad-Gita we come across the path of action which combines the rest of the three into one integrated whole in which a devotee has to live his life with a sense of supreme sacrifice, performing his actions with detachment, without any desire for the fruit of actions and offering them to God with pure devotion and total surrender.
Hindus have a very broader approach to the concept of God. The names that people give to Him are just mere reference points for the sake of our understanding. How can He have names, who is actually beyond all words and thoughts? He represent the loftiest ideal which mankind can aspire to achieve. He is the goal and reaching Him in our individual ways is the very purpose of our lives. Those who quarrel on his name are blind men who grope in darkness and go to the worlds of ignorance.
Truly the Brahman of Hinduism represents the Highest principle which the human mind can ever conceive of. He is not God of just one world or a few worlds, but represents the entire known and unknown Universe as well as the past, the present and the future that is yet to come.