Shaktism is a denomination of Hinduism that worships Shakti, or Devi -- the Hindu name for the Great Mother -- in all of her forms whilst not rejecting the importance of masculine and neuter divinity. In pure Shaktism, the Great Goddess, or Devi, is worshiped as nothing less than the highest divinity, Supreme Brahman Itself, the "one without a second," with all other forms of Divinity, female or male, considered to be merely her diverse manifestations.
In his seminal "History of the Shakta Religion," N. N. Bhattacharyya explained that "[those] who worship the Supreme Deity exclusively as a Female Principle are called Shakta. The Shaktas conceive their Great Goddess as the personification of primordial energy and the source of all divine and cosmic evolution. She is identified with the Supreme Being, conceived as the Source and the Spring as well as the Controller of all the forces and potentialities of Nature. Nowhere in the religious history of the world do we come across such a completely female-oriented system."
Alternative interpretations of Shaktism, however -- primarily those of Shaivite scholars, such as Satguru Sivaya Subramuniyaswami -- argue that the feminine manifest is ultimately only the vehicle through which the masculine Un-manifest Parasiva is ultimately reached. In this interpretation, the Divine Mother becomes something of a mediatrix, who bestows advaitic Moksha on those who worship Her. Thus, these Shaivite views often conclude that Shaktism is effectively a sub-denomination of Saivism, arguing that Devi is worshipped in order to attain union with Siva, who in Shaktism is the impersonal unmanifest Absolute. This remains a minority view in Shaktism proper, which considers Siva as an equal and inseparable aspect of Devi.
Shaktism as we know it today developed between the 4th and the 7th centuries CE in India. It was during this development that the many religious texts, known as the Tantras, were written. In a certain sense, one could consider oneself a Shakta (a devotee of Shakti), a Shaiva (a devotee of Shiva), and a Vaishnava (a devotee of Vishnu) all at the same time.
This form of Hinduism is strongly associated with Vedanta, Samkhya and Tantra Hindu philosophies and is ultimately monist, though there is a rich tradition of Bhakti yoga associated with it. The feminine energy (Shakti) is considered to be the motive force behind all action and existence in the phenomenal cosmos in Hinduism. The cosmos itself is Brahman, the concept of the unchanging, infinite, immanent and transcendent reality that is the Divine Ground of all being, the "world soul". Masculine potentiality is actualized by feminine dynamism, embodied in multitudinous goddesses who are ultimately reconciled in one.
The keystone text is the Devi Mahatmya which combines earlier Vedic theologies, emergent Upanishadic philosophies and developing tantric cultures in a laudatory exegesis of Shakti religion. Demons of ego, ignorance and desire bind the soul in maya (illusion) (also alternately ethereal or embodied) and it is Mother Maya, shakti, herself, who can free the bonded individual. The immanent Mother, Devi, is for this reason focused on with intensity, love, and self-dissolving concentration in an effort to focus the shakta'' (as a Shakti worshipper is sometimes known) on the true reality underlying time, space and causation, thus freeing one from karmic cyclism. A common hymn describing the 1000 names of Devi is the Lalita sahasranama.
Among the manifestations of Devi most favoured for worship by Shaktas are Kali, Durga, and Parvati. Durga is a War Goddess, and Kali is Goddess of Destruction, depicted as fierce and almost demonic.