The festival is marked by the tying of a rakhi, or holy thread by the sister on the wrist of her brother. The brother in return offers a gift to his sister and vows to look after her as she presents sweets to her brother. The brother and sister traditionally feed one another sweets.
It is not necessary that the rakhi be given only to a blood brother; any male can be "adopted" as a brother by tying a rakhi on the person, irrespective of whether he is cousin or a good friend. Indian history is replete with women asking for protection, through rakhi, from men who were neither their brothers, nor Hindus themselves.
The story of Rani Karnavati of Chittor and Mughal Emperor Humayun is the most significant evidence of this in history. During the medieval era, around the 15th century, there were many wars between the Rajputs, Mughals and Sultans. Rakhi at that time meant a spiritual binding and the protection of sisters was foremost. When Rani Karnavati, the widowed queen of the king of Chittor realised that she could in no way defend the invasion of the Sultan of Gujarat, Bahadur Shah, she sent a rakhi to Emperor Humayun. The Emperor was so touched by the gesture, that he abandoned an ongoing military campaign to ride to her rescue.
The rakhi may also be tied on other special occasions to show solidarity and kinship (not necessarily only among brothers and sisters), as was done during the Indian independence movement.