Cancer, in simple terms, occurs when cells divide in an uncontrolled manner. The human body is made up of different tissues and each tissue has its individual cellular structure. The cells in the tissues are lost through wear and tear and are constantly replaced by a process called cell division. Normally, cells divide - and produce the exact number of cells required - only when the body needs more cells. In cancer, however, cells divide for no apparent reason. In time, they form a mass of extra tissue and become big enough to be noticed as a growth or tumor.
A tumor can be benign - harmless - or malignant - bad. Benign tissues are rarely a threat to life, unless in a major organ. These do not invade the surrounding tissue and do not spread. They can usually be removed and do not come back. But malignant tumors grow into the surrounding tissue in a process called invasion. Cancer cells also break away from a malignant tumor and enter the blood stream or lymphatic system. This is how cancer spreads from the original tumor to other tissues and organs. The spread of cancer is called metastasis.