This is bound to be argued an oversimplification but:

What is a cancer? Uncontrolled cell growth. There're a lot of controls in a cell which try to prevent unrestricted growth, the defining point of progression to a cancer is the loss of those controls.

What is the difference between benign and malignant? Malignancy is the ability of the cancer to spread, though it can be difficult to tell that a cancer is capable of spreading without having observed it do so (however, in medicine nowadays we make diagnoses of "carcinoma-in-situ" which means a malignancy that hasn't spread yet). Evidence of spread can be local invasion (where a tumour progresses through the membrane surrounding a tissue), haematogenous (via the blood), lymphatic (through the lymphatic system) etc.

As for known causes I'll stay away; it's far too big a topic for me (and in parts quite debatable) - each type of cancer (and almost any tissue in the body can overgrow, though some very rarely) has its own list of factors, environmental, genetic etc. Smoking's always a good bet though.

I have friends and acquaintences who have worked on a prostate cancer treatment, which uses a tiny radioactive seed that is injected directly into the cancer, and the radioactivity kills the immediate surrounding tissue, also damaging or killing the cancer. In their tests, this seed has been VERY successful in treating prostate cancer. Nonetheless, they are not allowed to claim that it "cures" cancer. It treats that type of cancer, and knocks it down quite successfully, but that person always needs to be aware that the cancer can come back, there or elsewhere.

The point I am trying to make is that these scientists are careful to not ever use the word "cure," because there is no cure for cancer, just treatment. It may come back, it may not, you may die from being hit by a truck or a heart attack or something else that isn't cancer.