Each of our bodies is made up of million and millions of cells. Cells are the building blocks of tissue. There are lung cells, brain cells, blood cells, breast cells, and so on. The appearance and makeup of individual cells are unique to the tissue they compose. Brain cells are different from breast cells, and vice versa.
Most cells in the body normally divide to produce more cells when the body signals them to do so. However, occasionally cells fail to divide at the appropriate time, or cells may start dividing when no new cells are needed. If new cells start being produced when the body does not need them, the cells can form a mass of extra tissue. This extra tissue commonly is referred to as a growth, mass, or tumor. Breast tumors can be benign (not cancerous) or malignant (cancerous).
Malignant tumors are composed of cells that are growing out of control. These breast cancer cells also become undifferentiated, which means that they lose the distinguishing characteristics of the original breast tissue (i.e.: the breast cancer cells no longer have the features that characterize a normal breast cell as a breast cell). Malignant breast cancer cells can invade and injure nearby tissues and organs.
Breast cancer cells can also break away from the tumor and enter the bloodstream or lymphatic system. This is how breast cancer spreads to form secondary tumors in other parts of the body. The process by which cancer spreads to far away places in the body is called metastasis.