When two Hindu individuals utter the term “God,” their minds may not be referring to the same divine concept. For several seekers, their God may be the impersonal, formless, eternal, infinite, indescribable reality (Brahman, the closest Sanskrit term in scriptures for “God”) that Upanishads comprehensively talk about. For others, the term God could imply any of the following: the controlling authority or ruler of the universe, the creator, the Supreme Soul (Paramatma), the Personal God, the universal energy (shakti), the self (purified soul), or any other concept circulated by fellow humans that have realized God. … For the majority of Hindus, “God” refers to a personal or preferred form of the Absolute who has been selected from his various manifestations as described by the Puranas, epics, and legends.
The most common forms of God in current times, revered by almost all Hindus, include Vishnu and his Incarnations, Shakti and her manifestations, and Shiva. Traditionally, through widely circulated mythology, Vishnu is seen as our protector or the caretaker who comes to earth to live with us and establish righteousness. Out of his many visits to earth (incarnations), the ones in the form of Rama and Krishna are the most celebrated. Following the social impact of the devotee saints of the last millennium, Rama and Krishna have become the most loved forms of God for Hindus. Shakti, representing Brahman as primordial energy, is worshipped as the Mother of creation in her many forms like Durga and Kali. … Shiva is the form of Brahman who liberates souls and recycles the universe. Just like Vishnu, many consider him the Supreme Lord. His son Ganesha and incarnation Hanuman are also very dear to the masses.
Excerpted from Devotional Hinduism by M.S. Goel (2008), p. 2.
So, are Hindus monotheistic? Yes. Many devotees believe that all forms are manifestations of the same Brahman. Others create a hierarchy and see their favorite form (Personal God) as the Supreme God.
If you are trying to learn about the different forms of the Divine in mainstream Hinduism, the following chart from my blog may help:
(Feminine forms of God are shown in purple font; masculine forms are in saffron font. Names of the divine couples are underlined.)
Also, you can read this post.