How is cancer caused?

Depends: While we are aware of some well established causes of cancer, like cigarettes, there is much that remains a mystery.

Genetic mutations: All cancers result from the accumulation of genetic mutations. The genes have a natural tendency to mutate, making life as we know it possible, and there's background radiation and chemicals that are part of nature and inescapable. Specific cancers have various hereditary (inherit a bad gene) and environmental (mutagen) causes; anything can promote by making mutated cells divide.