There are several different types of breast cancer, which can develop in different parts of the breast.
Breast cancer is often divided into:
non-invasive breast cancer (carcinoma in situ) – found in the ducts of the breast (ductal carcinoma in situ, DCIS) and hasn't developed the ability to spread outside the breast. It's usually found during a mammogram and rarely shows as a breast lump.
invasive breast cancer – usually develops in the cells that line the breast ducts (invasive ductal breast cancer) and is the most common type of breast cancer. It can spread outside the breast, although this doesn't necessarily mean it has spread.
Other less common types of breast cancer include:
invasive (and pre-invasive) lobular breast cancer
inflammatory breast cancer
Paget's disease of the breast
It's possible for breast cancer to spread to other parts of the body, usually through the bloodstream or the axillary lymph nodes. These are small lymphatic glands that filter bacteria and cells from the mammary gland.
If this happens, it's known as secondary, or metastatic, breast cancer.