While it has been traditional to classify breast cancer types by what they look like under the microscope, it is more useful to think of breast cancer types by the things that they use to grow. If we can understand what makes a cancer grow, we can make it stop growing which is the goal of all cancer therapies. We can tell what a cancer cell is using to grow by what "receptors" it has on its cell surface.
When a cancer cell has estrogen or progesterone receptors on its surface, it is using a woman's own natural estrogen to grow. In that setting, blocking the estrogen receptor, or making estrogen unavailable to the cancer cell is effective therapy to stop cancer growth. In many cases, these "hormone receptor positive" cancers (also known as "ER/PR" positive) are so responsive to hormone blocking strategies that we don't have to use chemotherapy to control their growth.
On the other hand, if the cancer cell does not have these hormone receptors on its surface, it is called "hormone receptor negative" or "estrogen receptor (ER) negative." These cancers will not respond to hormone blocking strategies and we generally must use chemotherapy to control their growth.
The "Her-2-neu" receptor is another receptor that is important to a cancer cells growth. Only about 20% of all breast cancers have this receptor and when it is present, the cancer is able to grow more rapidly. Fortunately, we have a drug called Herceptin that is specifically designed to block this receptor and is very effective in controlling the growth of these "Her-2 positive" breast cancers. Because of its aggressive nature, this type of cancer is always treated with chemotherapy along with Herceptin.
When a cancer cell has neither the hormone receptors nor the Her-2-neu receptor, it is called "ER/PR/Her-2 negative" or "Triple negative" breast cancer. We don't really understand how these cancers grow but they will not respond to either homrone blocking drugs nor to Herceptin. They are always treated with chemotherapy and they are a bit more difficult to control so we tend to be fairly aggressive with them.