The mass or lump of extra tissue, called a tumor, can be benign or malignant. Benign tumors are not cancer. They can usually be removed, and in most cases, they don't come back. Most important, benign cells do not invade other tissues or spread to other parts of the body. Benign breast tumors are not a threat to life.
Malignant tumors are cancer.
The cancer cells grow and divide out of control. They can invade and damage nearby tissues and organs. Also, cancer cells can break away from a malignant tumor and enter the bloodstream or lymphatic system. That is how breast cancer spreads and forms secondary tumors in other parts of the body. The spread of cancer is called metastasis.