Sometimes this orderly process goes wrong. New cells form when the body doesn't need them, and old cells don't die when they should. These extra cells can form a mass of tissue called a growth or tumour. Not all tumours are cancer.
Tumours can be benign or malignant:
Benign tumours are not cancer: they are rarely life-threatening. Usually, benign tumours can be removed, and they seldom grow back. Cells from benign tumours don't spread to tissues around them or to other parts of the body.
Malignant tumours are cancer: they are generally more serious and may be life-threatening. Malignant tumours usually can be removed, but they can grow back. Cells from malignant tumours can invade and damage nearby tissues and organs.
Also, cancer cells can break away from a malignant tumour and enter the bloodstream or lymphatic system. That is how cancer cells spread from the original cancer (the primary tumour) to form new tumours in other organs. The spread of cancer is called metastasis.
Different types of cancer tend to spread to different parts of the body.