Breast cancer is a type of cancer that forms in tissues of the breast. This disease causes an uncontrolled growth of breast cells and a malignant tumor develops in the cells of the breast. In 2008, there were 182,450 new cases of breast cancer in women, and, although much more rare, 1990 new cases in males. In 2008, there were also over 40,000 deaths from the disease. 
>>>> About 1 in 8 women in the United States will have some form of breast cancer over their lifetime.
With Breast cancer, the cancer ussually forms in the the tubes that carry milk to the nipple (ducts) and glands that make milk (lobules). When genes that regulate the growth of cells change abnormally, cancer can occur.  Abnormal genes can change or mutate cells and, when this happens, tumors can occur. There are benign tumors which are not dangerous, and these are not considered cancerous.  Malignant tumors are dangerous and, when not treated, will grown beyond the original tumor to other parts of the body.
Resources:
http://www.cancer.gov/cancertopics/types/breast
http://www.cancer.gov/cancertopics/cancer-advances-in-focus/breast