Cancers start when one cell begins to grow and divide, rapidly and out of control.
It divides to give two cells, then four, eight, sixteen and so on until it forms a tumour - a growing clump of cancer cells. However, if that were all that cancer cells did, they would not be very dangerous. The real harm is done when they start to spread. Some tumours do not spread. As these benign tumours grow, the pressure of the surrounding body tissues makes it harder and harder for the tumour to expand, eventually forcing it to stop growing when it runs out of space.