This 3D animation from The Human Atlas describes breast anatomy and cancer.

The female breast is primarily composed of fatty tissue and mammary glands. Mammary glands drain into the lactiferous sinus, which connects them to the nipple. During pregnancy, mammary glands swell in order to accommodate milk production. This process is regulated by hormones.

Breast cancer is a malignant neoplasm of the mammary gland. It is one of the most common causes of cancer fatality in women. Early detection through regular breast self-examination and mammography is key. Through self-examination irregularities, such as lumps or masses, can be detected before the disease has had a chance to spread. The danger associated with all cancer, but especially with breast cancer, is that the disease can travel quickly once it has invaded the lymphatic system. The lymphatic system provides the perfect harbor for cancers to travel throughout the body.