Like Shiva, Kali possesses a third vertical eye in the middle of her forehead and is naked. Her sole garment is a long necklace of skulls and a belt made of severe hands. She is often portrayed as having four arms and a head but in this picture, she is shown as having ten heads and twenty arms. This is her Higher Form, named Mahakali (literally: Greater Kali). Her tongue hangs out craving for hot blood. In mythology, we find an explanation for this thirst. It started when Kali destroyed the demon Raktajiva, who had the boon of being born anew, a thousand times more powerful than before, each time a drop of his blood was shed. Kali, who knew the secret, pierced him with her lance and at once stuck her lips to the wound to drink all the blood as it gushed out, so that not a drop was left. Such was the paroxysm into which the taste of hot blood brought her that she fell into a fury and started killing all the demons around her.
Shiva, feeling that this destruction would upset the universal balance, threw himself down at her feet and barred her way with his body. That is why Kali is almost always represented with a foot upon Shiva's body.
This description by Kiranjyot.