There are billions and trillion of cells in each person's body. Cancer is a group of related diseases that all have to do with cells. Cancer occurs when abnormal cells grow and spread very fast. Normal cells grow, divide and know when to stop growing, by time they also die. But cancer cells continue on growing and dividing out of control and they don't die. They usually group together to form tumors. These tumors grow and become a lump of cancer cells that can destroy the normal cells around it and damage the body's healthy tissues. Which can make you very sick. Metastasis is when cancer tumors travels to other areas of the body where they can form a new tumor. This is how cancer spreads.