The flipside of ideath's description is that Kali is only a dark, single form of Parvati--Shiva's constant consort. In Hindu polytheism, all goddesses are in fact manifestations of Devi--The Great Goddess, despite their varying powers and characters. While shakti is essential for a male Hindu god to act, it is important to remember that the goddess relinquishes all of her power to the male god when she unites with him. Also, it deserves note that single goddesses often must use their powers to serve the gods. In one myth, Durga--an alternate form of Kali, was created by the gods in order to defeat Mahishusura the buffalo demon, so that they could resume their control over the universe.
Kali is however, a central figure in Tantrism; a strain of Hinduism which involves the subversion of its religious norms.