Cancer is a group of many different diseases that have some important things in common. They all affect cells, the body's basic unit of life. To understand different types of cancer, such as bladder cancer, it is helpful to know about normal cells and what happens when they become cancerous.

The body is made up of many types of cells. Normally, cells grow and divide to produce more cells only when the body needs them. This orderly process helps keep the body healthy. Sometimes cells keep dividing when new cells are not needed. These cells form a mass of extra tissue, called a growth or tumor. Tumors can be benign or malignant.

• Benign tumors are not cancer. They often can be removed and, in most cases, they do not come back. Cells in benign tumors do not spread to other parts of the body. Most important, benign tumors are rarely a threat to life.
• Malignant tumors are cancer. Cells in malignant tumors are abnormal and divide without control or order. These cancer cells can invade and destroy the tissues around them. Also, cancer cells can break away from a malignant tumor and enter the bloodstream or the lymphatic system. This process is the way cancer spreads from the original (primary) tumor to form new tumors in other parts of the body. The spread of cancer is called metastasis.