[benjijay]

Certainly there are myriad 'causes', and likely at least a few mechanisms if you get into specifics, but as a general rule a cancerous tumour is a group of cells that multiply faster than they die (or don't die at all), and consume resources without 'giving back' to the organ in which they reside - effectively the "I have outlived my designated usefulness" switch gets commented out and replaced with "multiply even harder, forever"