Kali is one of the main goddesses of Hindus. She is a very powerful goddess. Sometimes she is also called Chandi.
Durga and Kali killed many asuras (demons) who used to disturb human beings. Kali is a form of Durga that sprang from her head during Durga's battle with the demon Raktabeeja (who represents the power of thoughts, the power which constantly and unceasingly reproduce in the mind) whose blood when touch to the earth generated another Raktabeeja. To end this battle between Raktabeeja and Durga, Kali consumed every drop of blood until Raktabeeja bled out.
Kali is the highest form of the Supreme. Kali's name has two meanings; the first meaning is "dark" which means she represents the Void(brahman. The second meaning is "Time" which is the Destroyer of Everything.