Breast cancer is a disease where malignant cells form in breast tissues, often starting in the milk ducts or lobules and potentially spreading to other parts of the body. Symptoms can include a lump or thickening, changes in breast shape or skin (like dimpling or redness), nipple discharge, and a newly inverted nipple. It is the second most common cancer in women in the United States, and while it rarely affects men, it is a serious health concern for both.