Please explain what are the causes of cancer?

Like a computer prog: The instructions for how to run your body are like a computer program. When you make a copy of the program, there is a chance you will copy it wrong, and the program won't work any more. There is a smaller chance that the copy with malfunction and crash your computer. That, basically, is what cancer is. When your cells divide and one of them has a mistake in the dna copy that crashes your body.