Cancer. What really is the cause of this disease?

Five years ago, I was struggling to cope with depression and a feeling of emptiness after my beloved Dad succumbed to Leukemia (blood cancer).
Since then, life has not been easy but I had to come to acceptance that life must go on. I've been reading a lot of theories on the causes of cancer but I really don't come to full understanding on the real cause of cancer.

Question: What really causes cancer?