Sometimes abnormal cells develop in the breast tissue, forming a lump or tumor. This is the most common type of cancer in women. 

The breast has several lobes, which are divided into lobules and end in the milk glands. Tiny ducts run from the many tiny glands, connect together, and end in the nipple. Any tissue in the breast can be affected and it will destroy the nearby tissues also.  Usually the cancer arises from tissue that forms milk ducts. There are at least 15 different  kinds, depending on the site  of development . Both women and men can develop breast cancer, but it is very rare in men.

Breast cancer occurs more commonly in the left breast than the right and more commonly in the outer upper quadrant. The tumor may distort the shape of the breast or the texture of the skin as it becomes larger. It can be detected when it grow large enough to either be felt or seen on a mammogram.  

The cancer cells spread through the specialised channels in the breast called lymphatics to the lymph nodes to form tumors. It also spread or metasize to the other parts of the body through the blood stream. It spreads through the right side of the heart to the lungs, and eventually to the other breasts, the chest wall, liver, bone and brain. Spreading of the tumor to other parts of the body can cause death.

Breast cancer can be classified by histologic appearance and location of the lesion.