What Is Breast Cancer? Do you have all the breast cancer information you need? Most people have heard of it, but many do not realize what it is and how it occurs in the body. Like all types of cancer, the underlying cause is not always known. For some people, there is no family history while for others there is. For some people, being in great health hasn't stopped the cancer from coming on. Understanding what this cancer is and how it happens in the body can help anyone get through it.