Once a cell has transformed into a cancerous state, it begins to divide and multiply. In most cancers, these cells form a lump, or tumour, that can invade surrounding tissue. The tumour puts pressure on healthy cells around it, and may also secrete enzymes to break down that tissue.
By the time a tumour is detected, it will contain billions of cells. Cancerous cells produce growth factors that stimulate blood vessels to grow near them, enabling the tumour to build itself a nourishing blood supply. At later stages of the disease, cells break off the tumour and spread via blood or lymph vessels to seed secondary tumours in other parts of the body.