Key principles are, the law of the aurat (parts of the body that should not be viewed by or disclosed to the person who is not muhrim in Islamic religious law). The laws cover private parts, "aurat for Muslim men are starting from the knee to the center, while the Muslim women's aurat is the whole body except the face, the palms of her hands."
Your question about the cover for Muslim women's head, now you already know that the head is included for Muslim women's private parts. Why? For God commanded Muslim women should wear the veil when they were out of the house, so different from the pagan women and the female prostitute. For that same God to His Prophet commanded to convey this to his people God's announcement, which reads as follows:
"O Prophet, say to the wife-wives, daughters and wives of people believers:" Let them out of their garments throughout the body. "So that they might more easily to be recognized, therefore they are not alone. And Allah is Oft-Forgiving, Most Merciful. " (Al-Ahzab: 59)