Radioactive materials are composed of atoms that are unstable.  An unstable atom gives off excess energy until it becomes stable.  The energy the atom emits is radiation.  The process by which an atom changes from an unstable state to a more stable state by emitting radiation is called radioactivity. 

Radiation can be classified as either non-ionizing (low energy) or ionizing (high energy) radiation.  Types of non-ionizing radiation are ultraviolet light, visible light, infrared radiation, radio frequency radiation and microwaves.  Ionizing radiation  is given off by the sun (cosmic rays), radioactive materials, and high energy electronic devices (X-ray machines).  

There four major types of ionizing radiation.

• Alpha Particles are positively charged particles made up of two neutrons and two protons.  They are relatively heavy and slower moving than other radioactive emissions.  Alpha particles can be stopped  by a piece of paper or the dead outer layer of our skin.
• Beta Particles are negatively charged particles made up of an electron.  A beta particle is lighter and faster than an alpha particle and can be stopped by a thin piece of aluminium or a short span of air.
• Gamma Rays are short wavelength electromagnetic radiation emitted in the radioactive decay of an unstable atom.   Gamma radiation is highly penetrating and is stopped by lead.  
• X-rays are similar to gamma rays, but are generally lower in energy and less penetrating.  X-rays are emitted from processes outside the nucleus, while gamma rays originate inside the nucleus.    A few milimeters of lead can stop medical x-rays. 

Related Links:

American Nuclear Society Radiation Dose Chart
EPA Office or Radiation and Indoor Air Radiation Protection Division:  Ionizing Radiation Series
EPA Consumer Guide to Radon