Cancer is the uncontrolled growth and spreading of abnormal or damaged cells in the body. These cells can form tumors, which may be cancerous (malignant) or non-cancerous (benign). Cancerous tumors invade nearby tissues and can spread to other parts of the body, while benign tumors do not invade nearby tissues. Benign tumors, although they may be large, typically do not grow back after removal, whereas cancerous tumors can. Some benign tumors, such as those in the brain, can be life-threatening.