Radioactive substances are materials that emit radiation due to the decay of their atomic nuclei. This process occurs naturally in elements like uranium and radium, which can be found in the earth's crust. The radiation can take the form of alpha particles, beta particles, or gamma rays, and it can be harmful to living organisms if exposure is excessive. These substances are used in various applications, including medical treatments, such as radiation therapy for cancer, and in nuclear power plants for energy production. Safety measures are essential when handling radioactive materials to protect human health and the environment from potential hazards.