what is breast cancer ?

The term “breast cancer” refers to a malignant tumor that has developed from cells in the breast Usually breast cancer either begins in the cells of the lobules, which are the milk-producing glands, or the ducts, the passages that drain milk from the lobules to the nipple or in the stromal tissues.It is said to be always caused by a genetic abnormality . 
There are four main stages in the development of breast cancer . The major steps are indicated below :

STAGE 1 Cancer cells remain inside the breast duct, without invasion into normal adjacent breast tissue.
STAGE 2 Cancer  is confined to the breast .
STAGE 3 No tumor can be found in the breast, but cancer cells are found in the axillary lymph nodes (the lymph nodes under the arm)  OR  the tumor measures 2 centimeters or smaller and has spread to the axillary lymph nodes  OR  the tumor is larger than 2 but no larger than 5 centimeters and has not spread to the axillary lymph nodes.
STAGE 4 The tumor is larger than 2 but no larger than 5 centimeters and has spread to the axillary lymph nodes
STAGE 5 Cancer is found in axillary lymph nodes that are sticking together or to other structures, or cancer may be found in lymph nodes near the breastbone.
STAGE 6 The tumor may be any size and has spread to the chest wall and/or skin of the breast
STAGE 7 There may either be no sign of cancer in the breast or a tumor may be any size and may have spread to the chest wall and/or the skin of the breast  AND  the cancer has spread to lymph nodes either above or below the collarbone
STAGE 8 The cancer has spread — or metastasized — to other parts of the body.