One of the last sane

VERY briefly:
Cancer is a whole bunch of diseases in which there is cellular growth that is uncoordinated with, and invades or destroys, other tissues. Virtually any nucleated cell type in the body can undergo transformation to a cancer cell. Damage to DNA means that the cell reverts to an embryonic type, certain genes that are active in the embryo stage and are supposed to be switched off after that become expressed again, and cell multiplication and, in the case of malignant tumors, migration (invasion, metastasis), take off. Benign tumors grow and can destroy surrounding tissue by crushing it or its blood supply, while malignant tumors can invade surrounding tissue or metastasize (migrate) via the bloodstream, lymphatics or across cavities (thoracic, abdmonal etc). There are a bunch of cellular constraints that prevent uncontrolled growth and spread of cells in the normal individual. Cancer is considered a multi-step process i.e. DNA damage plus other factors promoting growth or suppressing the normal constraint mechanisms are required.

Cancers cause illness a lot of ways. They destroy essential tissues for one thing. Even benign tumors can destroy tissues by crushing them, especially if they grow inside a firm fibrous capsule or inside the cranium, so other tissues can't be pushed aside. Malignant tumors destroy by invading and replacing, for example if a malignant cell line in the bone marrow invades all the bone marrow and makes just one type of cell, you can die of a deficiency of the other types. Also if tumors grow fast they tend to outgrow their own blood supply and parts of them undergo necrosis (local cell death) which has nasty systemic effects; toxemia, wasting etc. They can also cause things like hemorrhages and perforations.

Malignant tumors that metastasize often have their worst effects at a site distant from their origin, for example they may spread to the lungs and replace so much of your respiratory tissue that you die, or spread to your kidneys and you die of renal failure.