Nipples are the pigmented projections found at the center of the breast tissue. In adult female mammals (such as a human woman), they contain the outlets of the milk ducts from the mammary glands. Humans nipples are surrounded by a lighter colored area called the areola.
More About Nipples
In addition to their biological funtion, nipples can also serve as an erogenous zone in people of all genders. Nipples contain erectile tissue that can respond to arousal as well as temperature. Some women can acheive orgasm through nipple stimulation.