The bladder is a hollow, muscular organ that acts rather like a balloon in that it expands to collect and store urine. Urine is made up of water and waste products that your body does not need. Urine is produced in the kidneys and travels down to the bladder via tubes called ureters. When the bladder is full enough to empty, or you feel the need to pass urine, the urine travels out of the bladder, and subsequently the body, via another tube called the urethra.
Your body is made up of tiny 'building blocks' called cells that repair and reproduce themselves in a controlled manner when other cells become damaged or die. If for some reason the cells divide and reproduce in an uncontrolled manner, resulting in more cells being made than are dying, they form into lumps. It is these lumps that are called tumours.
Tumours can be benign or malignant. Benign tumours grow but do not spread and therefore cause little trouble to the individual. Malignant tumours (or cancerous tumours) grow but also spread, often invading the surrounding tissues or structures. Sometimes cancer cells can break away from the original site and settle in other parts of the body where they may cause further damage. If this happens the cancerous tumours that then develop are called 'secondaries' or 'metastases'.