Fatty deposits on a woman's chest. They contain mammary glands which produce milk to feed babies, and have also evolved into a beauty/status symbol. Come in various sizes ranging from AAA (practically nonexistent) up to DDD and even higher. Generally, the bigger they are compared to the rest of a woman's body, the more attractive they are considered to be, although occasionally one will run across the rare man who prefers the little ones. The downside of having bigger ones is that they tend to sag after a woman reaches 30.
In today's breast-obsessed society, your breasts have to be at least a D-cup to be considered attractive.