Breast cancer is an uncontrolled growth of breast cells. To better understand breast cancer, it helps to understand how any cancer can develop.
Cancer occurs as a result of mutations, or abnormal changes, in the genes responsible for regulating the growth of cells and keeping them healthy. The genes are in each cell's nucleus, which act as the "control room" of each cell. Normally the cells in our bodies replace themselves though an orderly process of cell growth: healthy cells take over as old ones die out. But over time, mutations can "tun on" certain genes and "turn off" others in a cell. That changed cell gains the ability to keep dividing without control or order, producing more cells just like it and creating a tumor.
A tumor can be benign (not dangerous to health) or malignant (has the potential to be dangerous). Benign tumors are not considered cancerous: their cells are close to normal in appearance, they grow slowly and they do not invade nearby tissues or spread to other parts of the body. Malignant tumors are cancerous. Left unchecked , malignant cells eventually spread beyond the original tumor to other parts of the body.
For more information, check out http://www.breastcancer.org