The Devi Mahatmya, composed around the fifth or sixth century, is the text that made Shaktism a theology rather than a collection of cults. Its seven hundred verses tell three myths of the Goddess defeating demons the gods could not, and its argument is that the many goddesses are one, and that the one is the supreme reality. It holds a place in Shaktism comparable to that of the Bhagavad Gita elsewhere: it is recited entire, as an act of worship, and not merely read.
The tradition divides broadly into two streams. Srikula, and within it Srividya, is concerned with Lalita Tripurasundari, with the Sri Yantra, a diagram of nine interlocking triangles treated as the Goddess in geometric form, and with an elaborate and largely Brahminical practice of ritual and mantra. It is strongest in the south. Kalikula is concerned with the fierce forms, Kali and Tara above all, and is strongest in Bengal, Assam and Odisha.
Cutting across both is the distinction between the right-hand and left-hand paths. Left-hand practice deliberately uses what the ordinary religious order treats as polluting (meat, wine, sexual ritual, the cremation ground), on the principle that the Goddess is not partial to purity, and that the division between pure and impure is itself a limitation to be broken.
This is a real practice with a real literature, and it is also the part of Hinduism most consistently misrepresented by outsiders, who have generally taken the transgression for the point rather than for a method with a stated purpose. Most Shaktas, it should be said, follow the right-hand path and would not recognise the lurid version at all.