Quick Answer: Sindoor is a traditional red or orange-red vermilion powder applied along the parting of a married Hindu woman's hair, and the mangalsutra is a sacred necklace of black beads and gold tied around the bride's neck during the Hindu wedding ceremony. According to Wikipedia, sindoor is considered auspicious and serves as a visual marker of marital status — ceasing to wear it usually implies widowhood. The mangalsutra (or mangalasutra) literally means "auspicious thread" and is knotted around the bride's neck by the groom during the ceremony. Together, sindoor and the mangalsutra are the two most visible symbols of Hindu marriage, worn by married women across India, Sri Lanka, and Nepal.
Sindoor is made from traditional vermilion — a red pigment derived from cinnabar (mercury sulfide) or, more commonly today, from turmeric and slaked lime, which turns turmeric from yellow to red. The red color symbolizes female energy (shakti), power, love, and passion. In Hindu tradition, the red color is associated with Goddess Parvati, who is believed to protect the husband of any woman who wears sindoor. The application of sindoor is a central ritual called sindoor daan, where the groom applies sindoor to the bride's hair parting for the first time during the wedding ceremony.
The mangalsutra consists of black beads (kala manka) believed to offer protection against negative energies and the evil eye (nazar), and gold elements representing prosperity, purity, and divine blessing. The mangalsutra is tied by the groom with three knots, symbolizing the union of the couple for three lifetimes or the three aspects of marriage (physical, emotional, and spiritual). Regional variations include the Maharashtrian two-vati design, the South Indian thaali on yellow thread, the Bengali gold pendant on red and black thread, and the Punjabi elaborate gold pendant designs.