Most cells stay in the same place, die when told to and don't make more copies. Necessarily some of your cells don't obey one or more of these rules (you wouldn't last long if your blood cells weren't endlessly copied for example or if they stayed in one place). But if all three rules get broken you've got cancer.

The cancer I had when I was young, Hodgkin Lymphoma, is one extra rule broken because the lymphatic cells already don't stay in one place and makes extra copies of themselves - which is why it relatively often occurs in young people. One copying error and the "Die when told to" feature breaks in the new copies and now you've got cancer.