Cancer is a disease of uncontrolled proliferation of cells without any differentiation. Cell growth and differentiation is highly controlled and regulated in normal cell, but in cancer cells, there is breakdown of these regulatory mechanisms. Normal cells show a property called contact inhibition by virtue of which contact with other cells inhibits their uncontrolled growth. Cancer cells appear to have lost this property. As a result of this, cancerous cells just continue to divide giving rise to masses of cells called tumors. Cells sloughed from tumors reach distant sites through blood, and wherever they get lodged in the body, they start a new tumor there. This property is called metastasis.