Radiation is an electromagnetic wave on the same spectrum as radio-waves and visible light.
The type of radiation used in cancer therapy is ionizing radiation. Ionizing radiation has the property of causing biological effects in tissue through the deposition of energy. It is the damage of DNA that results in the death of tumor cells. The goal of radiation therapy is to lethally damage tumor cells, while not causing harm to normal tissue. To do this, as much normal tissue as possible is excluded from the radiation field.