They are called alpha and beta particles. Some radioactive materials are alpha emitters, and others are beta emitters. In addition, highly energetic rays called gamma rays are often emitted. Gamma rays are not material particles at all, but a form of pure energy very similar to x-rays, travelling at the speed of light. Gamma rays penetrate through soft tissue just as light shines through a window.
Beta particles have less penetrating power, travelling less than two centimeters in soft tissue.
Alpha particles have the least penetrating power, travelling just a few micrometers in soft tissue, equivalent to a few cell diameters. . . . Alpha particles, beta particles and gamma rays can do great harm to a living cell by breaking its chemical bonds at random and disrupting the cell's genetic instructions. Massive exposure to atomic radiation can cause death within a few days or weeks. Smaller doses can cause burns, loss of hair, nausea, loss of fertility and pronounced changes in the blood. Outside the body, alpha emitters are the least harmful, and gamma emitters are more dangerous than beta emitters.