The body is made up of tiny building blocks called cells. Usually, these cells divide to make new cells in a controlled way. Sometimes, this goes wrong and the cell becomes abnormal.
The abnormal cell keeps dividing and making more and more abnormal cells. These cells form a lump, which is called a tumour. Not all lumps are cancerous. Cancer cells sometimes break away from the primary cancer and travel through the blood or lymphatic system to other parts of the body. Cancer cells that spread and develop into a tumour somewhere else in the body are called a secondary cancer.