Cancer is unchecked cell growth. Mutations in genes can cause cancer by accelerating cell division rates or inhibiting normal controls on the system, such as cell cycle arrest or programmed cell death. As a mass of cancerous cells grows, it can develop into a tumor.
Disclaimer: I haven't had a proper science class in ages and I forgot most of the terms. I probably don't have the correct answer that you're looking for, but this is to the best of my ability.
Mutations in cells can cause cancer as it will compete with other cells for energy and space. It will continually multiply/rapidly grow and can't be rid of as its internal system of apoptosis was hindered during the mutation. Mutations can occur due to damages in the DNA.
