Every one of us is made up of cells, trillions of them. Cancer begins when one cell starts to grow uncontrollably. Healthy cells divide only when they need to. Cancerous cells, on the other hand, divide uncontrollably. The rapidly dividing cells push aside the healthy cells around them. They form a small cancer that at first is too small to see and then begins to grow beyond the organ where it started.
Cells divide when their genes tell them to. But if a gene has a mutation, it can tell a cell to divide when there's no reason for it to divide. Some early cancers stop growing and never spread. These are called benign tumors. However, many cancers invade tissue around them and spread to other parts of the body, these are malignant cancers.