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. … Some followers may worship a single favorite form of God throughout their life; others may choose to worship multiple forms in succession. Regardless of the aspect they adore, a hidden unity exists in their understanding of the Lord in popular Hinduism, which is more often panentheistic: Though the Divine pervades the entire creation, he, at the same time, is beyond the universe (Gita, 9: 4).

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.