Cells are produced by the body to replace dead or damaged ones. When cells unnecessarily multiply, they form tumors. There are a number of factors that can cause a cell to abort the normal cell cycle, resulting in excess cell division. These excess cells manifest themselves in two primary kinds of tumors, benign and malignant. Benign tumors are those that grow slowly, have defined walls and rarely spread to other parts of the body. Malignant tumors, are much more serious. While benign tumors may be taken out of the body with surgery, malignant tumors must be completely killed, usually with radiation or chemotherapy. Malignant tumors are classified by grades ranging from one to four. A grade one tumor is the least harmful, but should nonetheless be regarded very seriously. Grade four tumors reproduce rapidly and are the most threatening. For more information about how tumors are classified, contact a cancer specialist in your area.