Cancer stages...
Cells are programmed to grow, do a job, and die. A cell becomes cancerous when it ignores DNA programming rules. It becomes abnormal.
These abnormal cells replicate over and over and become a tumour. The tumour crowds out (bullies and kills) normal cells.
The tumour continues to grow and expand. It reaches your blood vessels and lymph nodes (kind of a superhighway to other organs)!
The cancer spreads to, and invades, other parts of your body. This primary cancer has now become a secondary cancer.