How can some cells become cancer?

Good question: Cancer describes a range of diseases in which one of your body's cells, or group of cells, undergo a change in genetic background that allows the cell(s) to live and divide regardless of the normal cues that would tell them to stop. This proliferation of careless cells hurts the local environment or that which it spreads to. Changes in dna are caused by environmental exposure plus your biology.

Genes & environment: Cancer arises from genetic changes in cells that lead to their uncontrolled growth. These changes arise from a complex interplay between our inherited genes and our environmental exposures. Hereditary cancers occur when defective genes are inherited, predisposing cells to become malignant. Environmental damage (smoking, hpv, radiation, diet) to cells also can cause these transformations.