Shaktism worships the Goddess as the supreme reality, and the claim it makes is stronger than it first sounds.
In much of Hinduism the goddesses are consorts: Lakshmi with Vishnu, Parvati with Shiva. Shaktism inverts the relation. Shakti is power itself, the active principle without which the gods are inert, and a stock image of the tradition shows Shiva as a corpse beneath the feet of Kali, conscious but incapable of acting until she moves. The Goddess is not the female aspect of a male god. She is what the male gods have to borrow.
She appears in a very wide range of forms, and the range is part of the point. Durga, who kills the buffalo demon the male gods together could not; Kali, black, garlanded with severed heads, a genuinely terrifying figure who is also, in Bengali devotion, addressed straightforwardly as Mother; Lalita Tripurasundari, the serene queen of the Srividya tradition; the ten Mahavidyas; and, at village level, thousands of local goddesses concerned with disease, fertility and the boundaries of the settlement, whose worship long predates any of the texts and continues alongside them.
The claim that this is the oldest religion of India, evidenced by prehistoric figurines and Indus valley artefacts, is made frequently and rests on interpretation rather than on demonstration. What is solid is that the Goddess becomes theologically central by the third or fourth century CE, and that the text which made her so is still the tradition's charter.