How do i know exactly what is cancer and how does it start?

Cells gone wild: In its' simplest form, a cancer is a line of cells that grow out of the normal control system of the human body. Cells develop and die on a regular basis daily within the human body. If a cell mutates, it usually dies, sort of a "poison pill" to keep cells within the normal cycle of the human body. Once a cell line becomes malignant, it grows unfettered by the normal system. I hope this helps.