Very good question everyone, very popular question too which is great! Cancer is caused by the cells in your body going nuts. It is a group of our own cells that have broken away from what they’re supposed to be doing and become parasitic. So its usually not the cancer cells themselves that kills people it’s usually that it spreads and prevents the body from doing its normal thing.
Regarding what actually causes it it can vary and it originates in the genes, and these can be inherited from birth or from something you expose yourself to after birth, for example smoking is a common cause for lung cancer, and radiation exposure can cause skin cancer.
The factors which cause cancer are termed ‘carcinogens’ and these cause the DNA inside a cell to become altered, or ‘mutated’, like in the X-Men films…in a much less friendly way. It causes the cell to grow without control and not ‘die’ when its supposed to.