A tumor is a mass of tissue that is formed by an accumulation of abnormal cells. Normally, the cells in the body age, die, and are replaced by new cells. With cancer and other tumors, something disrupts this cycle. Tumor (Wikipedia) cells grow, even though the body does not need them, and unlike normal old cells, they don't die. As this process goes on, the tumor continues to grow as more and more cells are added to the mass. There are two types of solid tumors: malignant (cancerous) and benign (noncancerous). Cancer (Wikipedia) is a disease in which cells, almost anywhere in the body, begin to divide uncontrollably. Cancerous tumors can invade nearby tissues in the body, and as they grow, some cells may travel to other places in the body to form other, “secondary” tumors.