For a brief overview of Vedanta go to the Wikipedia page.
The ancient Indian speculations on ‘the one and the many’ or on ‘sameness and difference’ compiled in the Upanishads (starting about 600 BC), came to be called Vedanta and understood as ‘the end of the Vedas’. The true origins of Vedantic lore are unknown, that lore being claimed by Brahmins to be God.
When the Vedas, allegedly created by Rishis, had begun to lose their charm and effectiveness due to gradual degradation by Brahmin priests as performers of stale, stifling and repressive ritual, a new breed of thinkers, allegedly kashtriyas (i.e. warriors), attempted to abstract and upgrade both their understanding of Veda and their observations of the world (just like the Buddha, their contemporary). They innovated generalizations, obvious to moderns, namely that all life appears to be of the same, i.e. of a single virtual (experienced by some as actual) origin (variously called Brahman, Atman or Prajpati ≈ God ≈ the Vital Force or the Godhead (viz. ‘Allah (formerly Eli) is One’)). They speculated that the one origin of all life, i.e. the Brahman exists as formless, i.e. nirguna, creation matrix that expresses/manifests via/as a zillion actual differentials, i.e. the saguna Brahman and of which the ‘gods’ are the first layer of elaboration or application.
In modern terminology one could describe the One (i.e. Brahman or Godhead as ‘sameness (in IT symbolized with the 0) giving rise to difference’ (in IT symbolized with the 1)) as an abstract creation algorithm (operating like an abstract Turing Machine) and the zillion differentials as fractal elaborations (of the one algorithm).
The Vedantins quickly recognised that the all-is-One (i.e. sameness with or without difference) view, nowadays called pantheism or pandeism, though virtually true, had no actual, hence practical, that is to say, political, value. Up to 10 different Vedantas eventually emerged, each one trying to turn the ‘oneness’ view into a much needed political (in India meaning ‘religious’) agenda. They catered for widely differing temperaments and needs, both personal and communal.
Today the stock in trade of most Hindu Gurus with an eye on foreign markets (like the now deceased ‘spiritual’ mass entertainers such as Ramana Maharshi, Krishnamurti and Satya Sai Baba) preach their own personal selection and variation of Vedanta’s ‘the one and the many’, i.e. ‘sameness and (or plus-minus) difference’ or ‘neti-neti’ (because frightened of ‘eti-eti’) speculations upon which they superimpose a practical theism (for instance the worship of Krishna) + morality (not found in the Upanishads), a rare few (like Krishnamurti) remaining agnostic or silent on the gods (as one-down elaborations). They treat Vedanta (and its very late but most influential political adaptation, the Bhagavadgita) as a kind of (everything goes) supermarket where one can freely pick and choose items to satisfy one’s personal hunger and taste for both ultimate/absolute and relative truth/being (Sanskrit: satya/sat).