Breast cancer is a type of cancer that begins in the breast tissue, most commonly in the cells lining the milk ducts or the lobules that produce milk. It is the most common cancer in women worldwide, although it can also occur in men, albeit less frequently. Breast cancer can vary widely in terms of its characteristics, treatment options, and prognosis, depending on factors such as the subtype of breast cancer, the stage of the disease, and individual patient factors.