How are tumor cells different from normal cells?

The big difference is growth. Normal cells stop growing when there are enough in one spot.

For example: When you get a cut, cells go to heal the wound, and the cells grow. But when the work is done, the cells stop growing.

But, cancer/tumor cells (not at a wound) grow and grow even when touching the first layers of muscle and skin. This would be what creates the lump. So, these bad cells are very overgrown and practically swollen cells.

Hope I helped.