* Cancer is not a disease, it's a family of diseases, with a wide spectrum of causes, symptoms, and treatments.
* Cancer is, fundamentally, a mutiny of your own cells against the rest of your body. Because cancer is fundamentally a part of you, it can shanghai some of the resources your body normally uses for itself, like blood vessel infrastructure. It can also masquerade as a healthy part of you, for example to your immune system. Hell, part of your immune system can become cancer (as in leukemia)!
* Cancer metastasizes, which means that even if you get rid of the main tumor, a few of those rebellious cancer cells can use your circulatory or lymphatic systems to start new rebel colonies elsewhere in your body, which means you could be fucked all over again. Treating this form of cancer can be an arduous, painful, sickness-inducing process of chemo- and/or radiotherapy.
We are closer than ever to "a cure for cancer" with things like immunotherapy treatments which can program your immune system to target the specific kind of cancer you have. Unfortunately, most of these need to be tailored per patient, which procedure is neither cheap nor routine. And it still doesn't always work for all cancers!