"Cancer" is when cells don’t grow and die as they normally do. Cancer is a broad term that covers a whole range of different cell behaviors. There are thousands of kinds of cancer. There are also many different kinds of breast cancer. Each has different treatment needs according to how they are behaving.

Why is cancer a problem? A tumor is a lump of cancer cells. It kills normal cells around it, causing damage (kind of like a rotten piece of fruit in a bowl of healthy fruit). It takes about 100 million cancer cells to make a 1cm-sized lump. It's difficult to confirm or spot cancer until it has grown to many millions of cells! (1).

If untreated, or if it grows unusually fast, cancer can spread outside of the breast. This is a problem because we can live without a breast, but we can’t live without working lungs, a liver, a brain, or bones. These four locations are where breast cancer usually spreads if it leaves the breast. This is called "metastatic" (pronounced meta-static) breast cancer, or "Stage IV" (stage 4) breast cancer.

If breast cancer leaves the breast, such as going to the liver, it doesn't mean the patient has "liver cancer." Instead the patient has breast cancer that has spread to the liver.

Each cancer has a different shape, that can be seen under a microscope. This is how we can tell what type of cancer it is, if it appears in a different part of the body.

There isn't just one kind of breast cancer. There are many different types, based on whether it uses hormones or certain proteins to grow (and genetic factors can also make an impact).

(1) Narod et al: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3320224/