What is cancer?

• Cancer occurs when cells in a part of the body begin to grow out of control. Normal cells divide and grow in an orderly fashion, but cancer cells do not. They continue to grow and crowd out normal cells. Although there are many kinds of cancer, they all have in common this out-of-control growth of cells.
• While cancer is always caused by a genetic "abnormality" (a "mistake" in the genetic material), only 5–10% of cancers are inherited from your mother or father. Instead, 90% of breast cancers are due to genetic abnormalities that happen as a result of the aging process and life in general.
• Breast cancer is the most common cancer to affect women. In 2004, about 216,000 new cases of invasive breast cancer were diagnosed in the United States, along with 59,390 new cases of non-invasive breast cancer.
• Invasive ductal carcinoma, or IDC, accounts for about 80% of all breast cancers.
• Less than 5 percent of all breast cancers occur in women under age 40. Women between the ages of 20 and 29 account for only 0.3% of breast cancer cases.

If you want more information, I've found these websites helpful:
Breastcancer.org
Komen Foundation
Imaginis
Dr. Susan Love Research Foundation